Is your SharePoint default CSS not loading?
Do you want to fix that, without the design of the site being distorted?
Follow these simple steps to resolve your issue!
– Then you will be taken to theme details page where click on “Try it out”
– Then you will be taken to theme preview page where click on “Yes, Keep it”
a. Change the master page in both dropdowns to “USAB/USAB”
b. Select “Reset all subsites to inherit this site master page setting” check box
c. Also select “Reset all subsites to inherit the theme of this site” check box.
d. Click OK
Note: Same settings highlighted in red in below screenshot for reference