I don’t understand your point. My point was that if you don’t care about other colors, you can simply stick with the default. The themes will automatically select the “best” color for your theme.

Then you can simply give it to them by allowing them to decide in the settings to do without multicolour highlights.
We prefer to steer clear of preferences like these because they significantly reduce the app’s consistency and polish.

I will not be an intensive user of multiple colours. I might use a second or even a third here and there, but that’s why I want to have the main colour that I don’t have to worry about determining. Precisely because I want it to be my main colour.
By the way: I like to switch light themes and don’t want to stick to just one just to minimise the annoyance
Would overriding the default highlight hotkey for a specific color resolve this issue for you? (For instance, yellow would now be your default highlight color for every theme?)