This behavior is by design - when you select not to load our default theme, the CSS that we're loading with it is not present on the page anymore, which leads to the observed by you issues. Please have in mind that if you want, you can always inspect the classes applied to particular elements on the page(e.g. using FireBug) and create your custom theme for Sitefinity. I believe this article
from our online documentation would be useful.
the Telerik team
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 Public Issue Tracking
system and vote to affect the priority of the items