Thank you for updating this post with your resolution.
Yes, by default it uses the Sitefinity basic theme. You can also access all of the files of this theme, widget templates, and resources by downloading the Sitefinity SDK the built in sitefinity theme is in sitefintiy SDK installation folder (by default its in C:/ProgramFiles/Telerik/SitefintiySDK and the theme is in Content/Resources/BasicFrontendTheme.zip) for theme registration follow this documentation..
I am having this problem as well (Sitefinity 6.1) and am confused about your solution. I've picked "No Template, start from scratch" when creating my page. I'm still getting the injected css.
"Figured it out. By default "Basic" is selected for everything it seems, just have to select it in the Page Template edit view." - I'm not sure how this applies or how to pick this option since I'm not sure what "Page Template edit view" is. When I'm on a page and click "Layout, I see that I can pick all kinds of column layouts. Down at the bottom I see that I've picked "No template". Where is the "Page Template edit view"?
Thanks for any help you can offer.
I finally figured out that picking "no theme" wasn't an option when picking "start from scratch with no template" - which seems kind of backwards to me. So, I made a template that was basically empty - just a content block - picked "no theme" and then dropped my html into the content block. Thanks for the help.