To clarify - you still have the ‘Default’ color scheme available (or can recreate it by just creating a new color scheme, which should have all the default choices filled in), but you can’t click the ‘Enable’ button on it?
If I recall, it looked like that prior as well though, there was no “default”.
I created a new scheme, made alterations, deleted that scheme, defaults never returned.
I can create a new color scheme and add what I believe were the default values, but I have no clue what they are now. If you could do a quick copy and paste of your values for the defaults, would help me out.