[UI] Some theme fixes (search bar, tour icon)

[arielj]

This PR fixes 2 issues with themes:

• the search bar had black text for all themes
• the tour icon on the top right corner had really bad contrast for nord-light theme (too light)

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

LGTM
I still cannot reproduce the black text on the search bar though. Tested all themes on 2.17.1 and they looks fine to me.