I’m having the same issue and have been looking into it. I put a detailed comment in Github with what I’ve found.
A short term fix is to create a CSS snippet that changes the value for the --text-selection variable. I’m using frost3. Here is the code to paste into a snippet:
:root {
--text-selection: var(--frost3);
}
The selection color will be quite different, but it’s at least visible everywhere. You could also try any of the the other theme colors or a CSS color value instead of a variable.