Hi,
some general infos, things to look at:
- to see changes to CSS file, you have to open another epub and eventually change back to the one you are looking at.
- please check the path & filenames they are casesensitive!
Maybe there is a misstype in the regular one, and so the bold is used.
Try to check a regular and bold text, side by side.
Is bold "bolder" then your regular?
- have you checked the brightness and contrast settings under options?
Mark