Menus: use surface variant in dark theme, surface in light theme

[jpnurmi]

Moving the previously hardcoded rgb(34,34,34) color into the color scheme makes it possible to:

• configure the background color of menus for high-contrast and flavors themes, and
• use the appropriate background color in custom widgets such as YaruAutocomplete.

Popup menus look the same as before in both light and dark themes. The only effective change is that surfaceVariant has now a different value in the dark theme. I sincerely hope that surfaceVariant was a safe choice for this but it's hard to know where it might be used... 😅

Before	After

Ref: ubuntu/yaru.dart#845
Ref: ubuntu/yaru.dart#668

[github-actions]

Unknown release type "Menus" found in pull request title "Menus: use surface variant in dark theme, surface in light theme".

Available types:

• feat: A new feature
• fix: A bug fix
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
• refactor: A code change that neither fixes a bug nor adds a feature
• perf: A code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
• ci: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs)
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit