Make the Debian icon in the upper-left corner of the web UI more visible

Right now the Debian logo on Salsa's front page looks like this:

Screenshot_20260129_044933

This made it a bit tricky for me to figure out my way back to my home page without re-typing a URL, since I wasn't able to see the icon to click it. Eventually I did see it, which is what inspired this request.

In case it matters, my system is using a light theme, and my web browser is Chrome (not Chromium, but full-fat Google Chrome) 144.0.7559.96.

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