Apply Dark Theme Color-invert to Main Menu Top Buttons

[fractile81]

trafficstars

Pulled the language button's color out of border and into a local variable so that can be inverted instead of the whole button. Applied "invert" filter for other top buttons.

[tobspr]

Nice, will have a look soon!

[dengr1065]

This change seems to work correctly: However, it might be even better to just invert the variable before it gets used, and specify it as background-color too:

$languageBorderColor: #222428;
@include DarkThemeOverride {
    $languageBorderColor: invert($languageBorderColor);
}
// ...
border: solid $languageBorderColor;
background-color: $languageBorderColor !important;

The scope of this change is Main Menu state, so there should be no regressions.