This is on purpose, it's intentionally recoloring the tabs in order to provide overt visual indication that the proxy is in use and the settings are being applied. I would be open to choosing a different color if the purple is obstructive, or possibly adding an option which disables the dynamic theming, but removing it entirely is not on the table. This is a feature, not a bug.

**Update**
OK I'll fire up a Gnome machine and re-evaluate some of the colors, that's a really widespread configuration as far as Linuxes go. I'm probably going to do a release late this month, perhaps early January, I expect it will be in by then.