Some users like myself like to be creative with their themes and Mods. However, Opera GX forces us to use certain values for the Lightness of both light and dark themes HSL colors.
Now I know it might make some things harder to read and it may not be as "pretty" but some of us prefer it this way.
For example: I'd like to have the yellow color from the Cyberpunk 2077 game cover, but I am limited to a booger yellow and a piss yellow for dark and light theme respectively instead of a "race car yellow".
Perhaps you could remove the L value of the HSL color of themes but add a warning to whoever that is trying to change the Lightness value; informing them that it may be hard of the eyes, makes things harder to read, etc.
I love that Opera GX gives a lot of liberty to the user and Im sure that it's one of the main reasons that people love this browser so giving such a constraint for something as simple as a color is quite baffling.
Sincerly,
Opera GX enjoyer, Software Developper
PS: Give your users the freedom they deserve. It can only do good.