Mention highlight color adjustment

[PlusInsta]

Is your feature request related to a problem? Please describe.
There is currently no way to customize the background highlight for messages that contain one or more mentions targeting the user.

Describe the solution you'd like
A dynamic "complementary color" system for such normally contrast-heavy elements.
In theory, rotating the hue dual back 120° already creates a complementary color, but there needs to be enough contrast to still read the message.
To demonstrate the concept, I created the interactive themer.
It is not feature-complete. By using it, you can tell that the shifted color can be both weaker and stronger than the base color. Some sort of "lightness compensation" might be needed for the offsets to make sense with any combination of hues.

Describe alternatives you've considered
This could also be done by adding a hidden variable, but I don't trust this to be a consistent enough solution to the problem and adds extra work to the user.
Maybe both can be done; a variable containing the complementary color calculation that can be overriden by the user.

Additional context
This is long, long overdue.