I have a little bit of a weird problem with the CMS.. after trying to make a module for the CMS. The UI generator start acting up..
The initial render of the Administration UI works fine. But now the tabs dont render properly anymore..
I have removed my mod, and even reinstalled Silverstripe to a blank databasen and the error persist.
Because Form is used to render all the forms on the site. It is the default template. If you want a custom template for your form you will need to override the renderWith() function on your form object
Note that as you have found out SS allows you to theme the cms from your themes folder so you need to be a bit careful with where you place elements.