The RadStyleSheetManager is used to minify and combine resources of the RadControls (or custom skins for them). If you want to minify your own style sheets you have to perform what is described in this
You can also try to check in your code if you are in DesignMode and disable the RadStyleSheedManager.
//TODO: Disable RadStyleSheedManager
What are these telerik styles that are breaking the backend, would it be possible to share a screenshot, please?
You can try adding your CSS to the frontend theme. It combines and loads all the CSS in one request, that way you can avoid using the stylesheet manager. You can find more info here: Themes and Working with themes.
With the RadStyleSheet manager you can also try the following: Set it not to be visible by default and then in code check if the page is in NOT in design mode and if this is true then set it to be visible.
//Set RadStyleSheedManager to be visible
Hi Atanas, I am already loading the CSS and images through the frontend theme. (as described in your links www.sitefinity.com/.../themes above) . When I load the CSS files this way, the site looks correct and the backend also works, however the files are NOT combined into a single request, (which is the reason I have been trying to get the stylesheetmanager to work). If you can tell me how I can combine the CSS files that are loaded via the frontend theme to be combined automatically into a single request that'll solve my problem. See the screenshots of "Working Backend" for a sample of what our backend page looks like, and the screenshot "Working CSS" too see how the CSS files are requested (as shown by firebug). As soon as I remove those files from the Frontend theme, and put a RadStyleSheetManager on the master page, when editing the EXACT same page as above it now looks like following screenshots:
As you can see when using the stylesheetmanager, my stylesheets are now combined into single request (which is what I'd like), BUT the backend page is unusable. I also have tried setting the visibility of the RadStyleSheetManager to false (by default) though this still doesn't seem to make any difference at all. The backend pages still render exactly the same way.