:root {
  --primary-color: #0077cc;
  --accent-color: #ff6600;
  --font-body: 'Roboto', sans-serif;

  /* for hw4_part4 */
  --bg-color: #fff;
  --text-color: #222;
  --header-bg: #002855;    /* UCSD blue for header/footer */
  --header-text: #FFD200;  /* UCSD yellow for header/footer*/
  --form-bg: #f0f8ff;

  /* section backgrounds */
  --section1-bg: linear-gradient(to right, #f0f8ff, #e6f0fa);
  --section2-bg: linear-gradient(to right, #e6f7ff, #d0ebff);
  --section3-bg: linear-gradient(to right, #fff9e6, #fef4cc);
  --section4-bg: linear-gradient(to right, #f3ffe6, #e6fadc);
  --section5-bg: linear-gradient(to right, #fce6ff, #f8d0fc);
}

/* add for hw4_part4 */
[data-theme="dark"] {
  --bg-color: #181c1f;
  --bg_ac-color: #fff;
  --text-color: #f2f2f2;
  --primary-color: #FFD200;
  --header-bg: #1f1f1f;
  --header-text: #FFD200;
  --form-bg: #242a2e;

  /* section backgrounds */
  --section1-bg: linear-gradient(to right, #232b33, #252e38);
  --section2-bg: linear-gradient(to right, #232e39, #20304b);
  --section3-bg: linear-gradient(to right, #332e1a, #2e291f);
  --section4-bg: linear-gradient(to right, #223322, #223322);
  --section5-bg: linear-gradient(to right, #322733, #3c2e39);
}

/* nav & header color match with footer(ucsd color)*/
.nav-header {
  background-color: var(--header-bg);
  color: var(--header-text);
  padding: 2rem 0;
  border-bottom: 1px solid #ccc;
}

.nav-container {
  max-width: 1200px;
  margin: 0 auto;
  padding: 0 2rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.site-title {
  font-size: 2rem;
  font-weight: bold;
  color: var(--header-text); /* UCSD Yellow */  
  text-transform: uppercase;
}

.nav-list {
  list-style: none;
  display: flex;
  gap: 2rem;
  padding: 0;
  margin: 0;
}

.nav-list li a {
  text-decoration: none;
  color: var(--header-text);
  font-weight: 500;
  font-size: 1rem;
  position: relative;
  padding-bottom: 0.3rem;
}

.nav-list li a:hover,
.nav-list li a.active {
  color: var(--bg-color);
  transition: color 0.3s ease-in-out;
}

[data-theme="dark"] .nav-list li a.active {
  color: var(--bg_ac-color);
}
[data-theme="dark"] .nav-list li a:hover {
  color: var(--bg_ac-color);
}

.nav-list li a::after {
  content: "";
  position: absolute;
  left: 0;
  bottom: 0;
  width: 0;
  height: 2px;
  background-color: var(--primary-color, #0077cc);
  transition: width 0.3s;
}

.nav-list li a:hover::after,
.nav-list li a.active::after {
  width: 100%;
  transition: width 0.3s ease-in-out;
}

@media (max-width: 768px) {
  nav {
    flex-direction: column;
  }
}

/* body */
body {
  font-family: var(--font-body);
  margin: 0;
  padding: 0;
  color: var(--text-color);
  background-color: var(--bg-color);
}

section {
  margin: 0;
  padding-top: 0;
  padding-bottom: 0;
  color: var(--text-color);
}

/* section backgrounds using variables */
/* section:nth-of-type(1) {
  background: linear-gradient(to right, #f0f8ff, #e6f0fa);
}

section:nth-of-type(2) {
  background: linear-gradient(to right, #e6f7ff, #d0ebff);
}

section:nth-of-type(3) {
  background: linear-gradient(to right, #fff9e6, #fef4cc);
}

section:nth-of-type(4) {
  background: linear-gradient(to right, #f3ffe6, #e6fadc);
}

section:nth-of-type(5) {
  background: linear-gradient(to right, #fce6ff, #f8d0fc);
} */

section:nth-of-type(1) {
  background: var(--section1-bg);
}

section:nth-of-type(2) {
  background: var(--section2-bg);
}

section:nth-of-type(3) {
  background: var(--section3-bg);
}

section:nth-of-type(4) {
  background: var(--section4-bg);
}

section:nth-of-type(5) {
  background: var(--section5-bg);
}

p, h2, h3 {
  color: var(--text-color);
}

.copyright-color{
  color: #fff !important; 
}

.hero {
  height: 100dvh;
  display: grid;
  place-items: center;
  background: linear-gradient(to bottom right, #0077cc, #a0d2eb);
  animation: fadeIn 1s ease-in-out;
  transition: transform 0.3s ease-in-out;
  color: var(--text-color);
}

.hero:hover {
  transform: scale(1.02);
}

@keyframes fadeIn {
  from { opacity: 0; }
  to { opacity: 1; }
}

/* image */
img,
picture {
  width: 100%;
  max-width: 180px;
  height: auto;
  object-fit: cover;
  object-position: center;
  display: block;
  border-radius: 0.5rem;
}

img:hover,
picture:hover {
  transform: scale(1.03);
  transition: transform 0.3s ease-in-out;
}

.responsive-image {
  width: 100%;
  max-width: 600px;
  height: auto;
  object-fit: cover;
  object-position: center;
  display: block;
  border-radius: 0.5rem;
}

.back-to-top {
  position: fixed;
  bottom: 2rem;
  right: 2rem;
  background-color: var(--primary-color);
  color: var(--text-color);
  padding: 0.5rem 1rem;
  border-radius: 0.5rem;
  text-decoration: none;
  font-weight: bold;
  display: none;
  z-index: 1000;
}

.back-to-top:hover {
  background-color: #005fa3;
}

@media (min-width: 768px) {
  .back-to-top {
    display: block;
  }
}

/* footer */
.site-footer {
  background-color: var(--header-bg);
  color: #fff;
  padding: 1rem 2rem;
  text-align: center;
}

.site-footer a {
  color: var(--header-text); /* UCSD Yellow */
  text-decoration: none;
  font-weight: bold;
}

.site-footer a:hover {
  text-decoration: underline;
}

.footer-content {
  max-width: 960px;
  margin: auto;
}



/* for contact */
.form-group {
  margin-bottom: 1rem;
  display: flex;
  flex-direction: column;
}

.form-group label {
  margin-bottom: 0.25rem;
  font-weight: bold;
  color: var(--text-color);
}

.form-group input,
.form-group textarea {
  padding: 0.5rem;
  border: 1px solid #ccc;
  border-radius: 0.25rem;
  font-size: 1rem;
  font-family: var(--font-body);
  background-color: var(--form-bg);
  color: var(--text-color);
  transition: background-color 0.3s ease-in-out, color 0.3s ease-in-out, border-color 0.3s ease-in-out;
}

.form-group textarea {
  resize: vertical;
  min-height: 120px;
}

.submit-button {
  background-color: var(--primary-color);
  color: var(--text-color);
  padding: 0.6rem 1.2rem;
  border: none;
  border-radius: 0.25rem;
  font-size: 1rem;
  font-weight: bold;
  cursor: pointer;
  transition: background-color 0.3s ease-in-out;
}

.submit-button:hover {
  background-color: #005fa3;
}

[data-theme="dark"] .submit-button {
  background-color: #003366;
  color: var(--header-text);
}

[data-theme="dark"] .submit-button:hover {
  background-color: #002244;
}

/* for hw4-part2 */
.form-group input:required,
.form-group textarea:required {
  border-left: 4px solid #ffa726;
}

.form-group input:optional,
.form-group textarea:optional {
  border-left: 4px solid #bdbdbd;
}

.form-group input:valid,
.form-group textarea:valid {
  border-color: #4caf50;
  background: #f1fff1;
}

[data-theme="dark"] .form-group input:valid,
[data-theme="dark"] .form-group textarea:valid {
  background: #1a2a1a;
  border-color: #4caf50;
}

.form-group input:invalid:not(:placeholder-shown),
.form-group textarea:invalid:not(:placeholder-shown) {
  border-color: #e53935;
  background: #fff4f4;
}

[data-theme="dark"] .form-group input:invalid:not(:placeholder-shown),
[data-theme="dark"] .form-group textarea:invalid:not(:placeholder-shown) {
  background: #3a1a1a;
  border-color: #e53935;
}

.form-group input:focus:invalid,
.form-group textarea:focus:invalid {
  outline: 2px solid #e53935;
}

.form-group input:focus:valid,
.form-group textarea:focus:valid {
  outline: 2px solid #4caf50;
}

.form-group input:placeholder-shown,
.form-group textarea:placeholder-shown {
  background: #fafafa;
}

[data-theme="dark"] .form-group input:placeholder-shown,
[data-theme="dark"] .form-group textarea:placeholder-shown {
  background: var(--form-bg);
}

/* Contact form and address bg/text for theme */
.contact-form,
.contact-address {
  background: var(--form-bg);
  color: var(--text-color);
}



.theme-switch-wrapper {
  display: flex;
  align-items: center;
  justify-content: flex-end;
  gap: 1rem;
  margin-top: 1.5rem;
  margin-bottom: 1.5rem;
}

.theme-switch-label {
  font-size: 1.1rem;
  font-weight: 700;
  color: #3397FF;
  letter-spacing: 1px;
  margin-right: 10px;
}

.theme-switch {
  position: relative;
  display: inline-block;
  width: 62px;
  height: 32px;
  vertical-align: middle;
}

.theme-switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

.slider {
  position: absolute;
  cursor: pointer;
  top: 0; left: 0; right: 0; bottom: 0;
  background: #ccc;
  border-radius: 40px;
  transition: background 0.3s;
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 7px;
  font-size: 0.95rem;
}

.switch-text {
  position: absolute;
  color: #fff;
  font-size: 0.8rem;
  top: 8px;
  width: 28px;
  text-align: center;
  z-index: 2;
  pointer-events: none;
}
.slider .on { left: 2px; }
.slider .off { right: 2px; }

.slider:before {
  position: absolute;
  content: "";
  height: 24px;
  width: 24px;
  left: 5px;
  top: 4px;
  background-color: #fff;
  border-radius: 50%;
  transition: transform 0.3s;
  box-shadow: 0 2px 5px rgba(0,0,0,0.15);
  z-index: 3;
}

.theme-switch input:checked + .slider {
  background: #3397FF;
}

.theme-switch input:checked + .slider:before {
  transform: translateX(28px);
}

.theme-switch input:checked + .slider .on { color: #fff; opacity: 1; }
.theme-switch input:checked + .slider .off { color: #fff; opacity: 0.5; }
.theme-switch input:not(:checked) + .slider .on { color: #fff; opacity: 0.5; }
.theme-switch input:not(:checked) + .slider .off { color: #fff; opacity: 1; }

/* Resume/Skills Table Styles */
.skills-table {
  width: 400px;
  margin: 2rem 0;
  border-collapse: collapse;
  font-size: 1.1rem;
  background: rgba(255,255,255,0.7);
}

.skills-table th,
.skills-table td {
  border: 1px solid #bbb;
  padding: 0.5rem 1.2rem;
  text-align: left;
}

.skills-table th {
  background: #ffe082;
  font-weight: bold;
}

.skills-table tr:nth-child(even) td {
  background: #fff9e6;
}

/* Dark mode for table */
[data-theme="dark"] .skills-table th,
[data-theme="dark"] .skills-table td {
  background: #1e2328;
  color: #fff;
  border-color: #444;
}

[data-theme="dark"] .skills-table th {
  background: #23272b;
  color: #FFD200;
}

[data-theme="dark"] .skills-table tr:nth-child(even) td {
  background: #292f36;
}
[data-theme="dark"] .skills-table tr:nth-child(odd) td {
  background: #23272b;
}

.skills-table tr:hover td {
  background: #e8f0ff;
  color: #222;
}
[data-theme="dark"] .skills-table tr:hover td {
  background: #3a4562;
  color: #FFD200;
}