Bold in "show-paren-match" causes text displacement with proportional fonts

Hi, and thanks for those amazing themes.

Until I read about them in a recent blog post, I had given up hope of ever finding a light theme I'd be completely happy with.

I have a slight problem with buffers using a proportional font: the font show-paren-match has a bold weight, which is not a problem with monospace fonts, but with proportional fonts it causes text to move slightly laterally when the face gets applied or disapplied.

Given that this face is applied temporarily depending on the position of the point, the effect is quite visible.

Is it fixable, or is this attribute there for a good reason?