What problem does this address?

Currently, users are free to choose text and background colors that do not meet minimum color contrast standards. The editor displays a warning, but this allows the user to make the choice before warning them it would be inaccessible.

What is your proposed solution?

Once the user chooses a background color the editor should hide all inaccessible text color choices and add a "show all colors" button to restore them. A rough mockup:

Once the user clicks the "show all colors" button, the editor should display the message explaining why the user's choices were constrained and suggesting they only use recommended colors. Again, a rough mockup:

Additionally, I would suggest adding a setting that allows theme authors to disable the "show all colors" button. This way, themes that are marketed as accessible can make firmer guarantees about accessibility, and users seeking out accessible themes will have fewer avenues available allowing them to inadvertently create accessibility issues.