Rollup merge of #112274 - GuillaumeGomez:migrate-gui-test-color-11, r=notriddle

Migrate GUI colors test to original CSS color format

Follow-up of https://github.com/rust-lang/rust/pull/111459.

The update for `browser-ui-test` version is because for hex color conversions, it used a precision of 1 instead of 2, which was problematic.

r? `@notriddle`

src/ci/docker/host-x86_64/x86_64-gnu-tools/browser-ui-test.version

@@ -1 +1 @@
-0.16.5
+0.16.6

tests/rustdoc-gui/sidebar-mobile.goml

@@ -73,16 +73,16 @@ define-function: (
 
 call-function: ("check-colors", {
     "theme": "ayu",
-    "color": "rgb(197, 197, 197)",
-    "background": "rgb(20, 25, 31)",
+    "color": "#c5c5c5",
+    "background": "#14191f",
 })
 call-function: ("check-colors", {
     "theme": "dark",
-    "color": "rgb(221, 221, 221)",
-    "background": "rgb(80, 80, 80)",
+    "color": "#ddd",
+    "background": "#505050",
 })
 call-function: ("check-colors", {
     "theme": "light",
-    "color": "rgb(0, 0, 0)",
-    "background": "rgb(245, 245, 245)",
+    "color": "black",
+    "background": "#F5F5F5",
 })