The theme download doesn't show that it's compatible with 3.3.x
https://moodle.org/plugins/theme_eguru
And as you've discovered the theme is making an http call for fonts ... which on a https site would complain about mixed.
Suggest contacting makers of the theme and inquire about a 3.3.x version and the issue related to making http call rather than https.
'spirit of sharing', Ken