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"> {#- -#}