Migrate summary toggle filter to CSS variable

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

@@ -1521,6 +1521,7 @@ details.rustdoc-toggle > summary::before {
 	display: inline-block;
 	vertical-align: middle;
 	opacity: .5;
+	filter: var(--toggle-filter);
 }
 
 details.rustdoc-toggle > summary.hideme > span,

src/librustdoc/html/static/css/themes/ayu.css

@@ -21,6 +21,7 @@ Original by Dempfi (https://github.com/dempfi/ayu)
 	--right-side-color: grey;
 	--code-attribute-color: #999;
 	--toggles-color: #999;
+	--toggle-filter: invert(100%);
 	--search-input-focused-border-color: #5c6773; /* Same as `--border-color`. */
 	--copy-path-button-color: #fff;
 	--copy-path-img-filter: invert(70%);
@@ -158,10 +159,6 @@ body.source .example-wrap pre.rust a {
 	background: #333;
 }
 
-details.rustdoc-toggle > summary::before {
-	filter: invert(100%);
-}
-
 .module-item .stab,
 .import-item .stab {
 	color: #000;

src/librustdoc/html/static/css/themes/dark.css

@@ -16,6 +16,7 @@
 	--right-side-color: grey;
 	--code-attribute-color: #999;
 	--toggles-color: #999;
+	--toggle-filter: invert(100%);
 	--search-input-focused-border-color: #008dfd;
 	--copy-path-button-color: #999;
 	--copy-path-img-filter: invert(50%);
@@ -89,10 +90,6 @@ body.source .example-wrap pre.rust a {
 	background: #333;
 }
 
-details.rustdoc-toggle > summary::before {
-	filter: invert(100%);
-}
-
 #titles > button:not(.selected) {
 	background-color: #252525;
 	border-top-color: #252525;

src/librustdoc/html/static/css/themes/light.css

@@ -16,6 +16,7 @@
 	--right-side-color: grey;
 	--code-attribute-color: #999;
 	--toggles-color: #999;
+	--toggle-filter: none;
 	--search-input-focused-border-color: #66afe9;
 	--copy-path-button-color: #999;
 	--copy-path-img-filter: invert(50%);