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?
This is what the current color scheme area in the ACP looks like (so no “default” is there):
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.