DOC: Update doc theme

Replace default secondary color with something that better matches
Unidata colors. Also remove and clean up some of the rest of the custom
theming, including making dark mode work.

docs/_static/theme-unidata.css

@@ -1,6 +1,17 @@
 /* Define "Unidata Blue" RGB values */
 html {
-    --unidata-blue-rgb: 6, 119, 143;
+    --unidata-blue-rgb: rgb(6, 119, 143);
+    --unidata-teal-rgb: rgb(69, 166, 169);
+}
+
+html[data-theme="light"] {
+    --pst-color-primary: var(--unidata-blue-rgb);
+    --pst-color-secondary: var(--unidata-teal-rgb);
+}
+
+html[data-theme="dark"] {
+    --pst-color-primary: var(--unidata-blue-rgb);
+    --pst-color-secondary: var(--unidata-teal-rgb);
 }
 
 /* Header and heading font style */
@@ -10,32 +21,20 @@ html {
 
 /* Header colors */
 .bd-header {
-    background: rgb(var(--unidata-blue-rgb)) !important;
+    background: var(--unidata-blue-rgb) !important;
 }
 
-.theme-switch-button {
-    border-color: rgb(var(--unidata-blue-rgb)) !important;
-}
-
-.bd-header .navbar-nav>.nav-item>.nav-link, 
+.bd-header .navbar-nav>.nav-item>.nav-link,
 .bd-header .dropdown-toggle,
 .bd-header .pst-navbar-icon,
 .bd-header .btn {
     color: #fff !important;
- }
-
-.version-switcher__button {
-    border-color: #fff !important;
 }
 
 .search-button {
     padding: 0 0 0;
 }
 
-.navbar-nav .dropdown-menu {
-    background-color: var(--pst-color-background);
-}
-
 /* Increase contrast of links in code snippets */
 div[class^="highlight"] a {
     background-color: rgb(var(--unidata-blue-rgb), 0.2);