You can create a new theme and customize the selectors you want, and then apply the new theme to your site. That way, the site will have the general look of a SharePoint site, but with the modifications you want.
The way to do it:
- Open your site in FP
- Format | Theme
- At the very bottom of the theme task pane select "Create new Theme”
- You’ll get the “Customize” theme window
- Click on “Text”
- Click on “More Text Styles”
- From the list, choose the selector you want
- Modify (modify the attribute(s) you are interested in for that selector)
- Repeat for each selector you want to modify
- | OK | OK
- Save as “MyTheme” |OK
- Apply “MyTheme” to your SharePoint site
When you select customize a theme, goto the Text panel and click on “More Styles” – there you can edit all class styles (ie .ms-xxxxx).
Note, the theme does not replace the existing (ows.css on an STS site) style sheet, but instead acts as a modifier for it.