Use =white() as expression for the Background Color setting.
You can change text to bold by using the Text Format setting and use ='<B>' as expression for that setting.
If you want to change these settings only for total levels then you can use the dimensionality() or secondarydimensionality() function. Something like =if(Dimensionality()=0, '<B>')
why don't you just copy your existing Expression (with the formating you want to ovetake)?
select your Expression, right mouse click, copy
right mouse again and paste, Change Expression to your liking
if paste is grayed, Position Cursor elsewhere in expressions and right mouse, paste should be available