Migrate GUI colors test to original CSS color format

Author: GuillaumeGomez

Diff for: tests/rustdoc-gui/codeblock-tooltip.goml

@@ -109,19 +109,19 @@ define-function: (
 
 call-function: ("check-colors", {
     "theme": "ayu",
-    "background": "rgb(15, 20, 25)",
-    "color": "rgb(197, 197, 197)",
-    "border": "rgb(92, 103, 115)",
+    "background": "#0f1419",
+    "color": "#c5c5c5",
+    "border": "#5c6773",
 })
 call-function: ("check-colors", {
     "theme": "dark",
-    "background": "rgb(53, 53, 53)",
-    "color": "rgb(221, 221, 221)",
-    "border": "rgb(224, 224, 224)",
+    "background": "#353535",
+    "color": "#ddd",
+    "border": "#e0e0e0",
 })
 call-function: ("check-colors", {
     "theme": "light",
-    "background": "rgb(255, 255, 255)",
-    "color": "rgb(0, 0, 0)",
-    "border": "rgb(224, 224, 224)",
+    "background": "white",
+    "color": "black",
+    "border": "#e0e0e0",
 })