Fonts not changing when I change themes for any site in Sharepoint 2010. The colors are changing cor

I am trying to improve the look and feel of the sites..IE: the headings are not bolded and it is hard to tell the difference between the webpart lists.

When I go to themes and change the fonts…preview>>>nothing changes with the fonts at all…only the colors. I click apply…qnd still no change in fonts, only colors.

Help!