Pick themes on settings page, not every page

This hides the paintbrush icon on most pages by default, in preference
for the settings on the settings page.  When loading from a local file,
and not in mobile view, continue to show the theme picker. That's
because some browsers limit access to localStorage from file:/// URLs,
so choosing a theme from settings.html doesn't take effect.

src/librustdoc/html/static/css/rustdoc.css

@@ -1755,6 +1755,12 @@ details.rustdoc-toggle[open] > summary.hideme::after {
  padding-top: 0px;
  }
 
+ /* Space is at a premium on mobile, so remove the theme-picker icon. */
+ #theme-picker {
+ display: none;
+ width: 0;
+ }
+
  .rustdoc {
  flex-direction: column;
  }
@@ -1873,12 +1879,6 @@ details.rustdoc-toggle[open] > summary.hideme::after {
  height: 100%;
  }
 
- nav.sub {
- width: calc(100% - 32px);
- margin-left: 32px;
- margin-bottom: 10px;
- }
-
  .source nav:not(.sidebar).sub {
  margin-left: 32px;
  }
@@ -2075,11 +2075,6 @@ details.rustdoc-toggle[open] > summary.hideme::after {
  border: 0;
  }
 
- #crate-search + .search-input {
- width: calc(100% + 71px);
- margin-left: -36px;
- }
-
  #theme-picker, #settings-menu {
  padding: 5px;
  width: 31px;

src/librustdoc/html/static/js/main.js

@@ -129,10 +129,15 @@ function hideThemeButtonState() {
 
 // Set up the theme picker list.
 (function () {
+ if (!document.location.href.startsWith("file:///")) {
+ return;
+ }
  var themeChoices = getThemesElement();
  var themePicker = getThemePickerElement();
  var availableThemes = getVar("themes").split(",");
 
+ removeClass(themeChoices.parentElement, "hidden");
+
  function switchThemeButtonState() {
  if (themeChoices.style.display === "block") {
  hideThemeButtonState();

src/librustdoc/templates/page.html

@@ -96,7 +96,7 @@
  {%- endif -%}
  </a> {#- -#}
  <nav class="sub"> {#- -#}
- <div class="theme-picker"> {#- -#}
+ <div class="theme-picker hidden"> {#- -#}
  <button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"> {#- -#}
  <img width="18" height="18" alt="Pick another theme!" {# -#}
  src="{{static_root_path|safe}}brush{{page.resource_suffix}}.svg"> {#- -#}