Add "minimum contrast threshold" setting/feature

[carterworks]

Potentially related to #3415

Is your feature request related to a problem? Please describe.

From @jvns's "Terminal colours are tricky":

A bunch of terminals (iTerm2, tabby, kitty’s text_fg_override_threshold, and folks tell me also Ghostty and Windows Terminal) have a “minimum contrast” feature that will automatically adjust colours to make sure they have enough contrast.

Here’s an example from iTerm. This ngrok accident from before has pretty bad contrast, I find it pretty difficult to read:

This problem can also happen in Wezterm. With the "Selenized Light (Gogh)" theme:

Describe the solution you'd like

With “minimum contrast” set to 40 in iTerm, it looks like this instead:

I didn’t have minimum contrast turned on before but I just turned it on today because it makes such a big difference when something goes wrong with colours in the terminal.

Describe alternatives you've considered

I could edit my theme, but an automatic solution might be more helpful.