Default to light theme is JS is enabled but not working

Author: GuillaumeGomez

src/librustdoc/html/static/css/noscript.css

@@ -34,7 +34,7 @@ nav.sub {
 	in rustdoc.css */
 
 /* Begin theme: light */
-:root {
+:root, :root:not([data-theme]) {
 	--main-background-color: white;
 	--main-color: black;
 	--settings-input-color: #2196f3;
@@ -140,7 +140,7 @@ nav.sub {
 
 @media (prefers-color-scheme: dark) {
 	/* Begin theme: dark */
-	:root {
+	:root, :root:not([data-theme]) {
 		--main-background-color: #353535;
 		--main-color: #ddd;
 		--settings-input-color: #2196f3;

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

@@ -2315,8 +2315,14 @@ in src-script.js and main.js
 	tooling to ensure different themes all define all the variables. Do not
 	alter their formatting. */
 
+/*
+About `:root:not([data-theme])`: if for any reason the JS is enabled but cannot be loaded,
+`noscript` won't be enabled and the doc will have no color applied. To do around this, we
+add a selector check that if `data-theme` is not defined, then we apply the light theme
+by default.
+*/
 /* Begin theme: light */
-:root[data-theme="light"] {
+:root[data-theme="light"], :root:not([data-theme]) {
 	--main-background-color: white;
 	--main-color: black;
 	--settings-input-color: #2196f3;