Rollup merge of #112064 - GuillaumeGomez:migrate-gui-test-color-9, r=notriddle

Migrate GUI colors test to original CSS color format

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

The `browser-ui-test` update is a fix when converting the alpha value to hex format. More information [here](https://github.com/GuillaumeGomez/browser-UI-test/pull/511).

r? ````@notriddle````

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

@@ -1 +1 @@
-0.16.4
+0.16.5

tests/rustdoc-gui/target.goml

@@ -25,11 +25,11 @@ call-function: ("check-style", {
 })
 call-function: ("check-style", {
     "theme": "dark",
-    "background": "rgb(73, 74, 61)",
-    "border": "rgb(187, 116, 16)",
+    "background": "#494a3d",
+    "border": "#bb7410",
 })
 call-function: ("check-style", {
     "theme": "light",
-    "background": "rgb(253, 255, 211)",
-    "border": "rgb(173, 124, 55)",
+    "background": "#fdffd3",
+    "border": "#ad7c37",
 })