Test: Theme colors in webviews

[mjbvz]

Test for #41785

Complexity: 1

• Windows @dbaeumer
• Mac @misolori
• Linux @alexandrudima

Summary
Webviews can now access theme colors using css variables. These variable names are prefixed with vscode and replace the . with -. For example editor.foreground becomes var(--vscode-editor-foreground)

Test
Ensure that you can use these variables in a webview (you can use the cat coding example for this)

Check that you can:

• Access all theme colors as css variables
• That the variable values are updated properly when the theme changes