Migrate GUI colors test to original CSS color format

Author: GuillaumeGomez

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",
 })