rustdoc: revert tooltip background color on light theme to readable

text #fff on background #fdffd3 fails the [WCAG color contrast checker], and
seems like a mistake in 16b5590.

[WCAG color contrast checker]: https://accessibleweb.com/color-contrast-checker/

Author: notriddle

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

@@ -64,7 +64,7 @@
 	--test-arrow-hover-background-color: #4e8bca;
 	--target-background-color: #fdffd3;
 	--target-border-color: #ad7c37;
-	--tooltip-background-color: #fdffd3;
+	--tooltip-background-color: #000;
 	--tooltip-color: #fff;
 	--kbd-color: #000;
 	--kbd-background: #fafbfc;

src/test/rustdoc-gui/codeblock-tooltip.goml

@@ -146,7 +146,7 @@ call-function: ("check-colors", {
 })
 call-function: ("check-colors", {
     "theme": "light",
-    "background": "rgb(253, 255, 211)",
+    "background": "rgb(0, 0, 0)",
     "color": "rgb(255, 255, 255)",
     "border": "rgb(224, 224, 224)",
 })