TL; DR, OMGF Pro is compatible with 99% of themes. OMGF might not be in some cases.
OMGF is capable of parsing the HTML document for the presence of stylesheets loaded from the Google Fonts API (fonts.googleapis.com/css)
OMGF Pro "digs deeper" and can process Google Fonts present in:
- Inline (wrapped in <style> blocks) @font-face and @import statements,
- @font-face and @import statements in local stylesheets loaded by theme and plugins,
- WebFont Loader,
- Stylesheets loaded from Google Fonts' Early Access API (fonts.googleapis.com/earlyaccess)
Want to know if your theme has been tested with OMGF Pro, take a look at this list.