If the line is 0pt, how does the checkbox "check"?
Can you show screen shots?
I've just noticed it is possible to create the same kind of checkbox from scratch in 12. You have to draw lines manually for the borders/engraved effect, but it works. So yay not having to do text entry fields and button scripts. The problem is that converted ones do not work.
This may have something to do with the padding in the Theme CSS...
I usually deal with this type of a checkbox situation by using a button instead. Button may have an "X" as text... By default the text color matches the button so it is invisible. Then I use conditional formatting to change the color of the text or button. Below are some examples or the formula to use. This approach gives you more creative flexibility then a checkbox field alone would.
Button ( sets the XFIELD ) ...
Case ( XFIELD = X ; "" ; "X" )
Conditional Formatting ( make text color visible color ) ...
XFIELD = "X"