The Sitefinity's default theme loads some basic css when the pages are rendered. If you do not want these css files to be loaded you can go to the Sitefinity backend
and then click on the Design -> Page Templates
and click on your template
in order to open the page template in edit mode. The please click on the Theme
tab on the right-hand side of the page and from the dropdown select "no theme".
This will minimize the css introduced from Sitefinity into your pages, however there will still be a little amount that we keep injecting in order to keep page content okay. Here is also a short video
for your reference demonstrating the above steps.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items