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.

Check attached file for more info..

Tried something..

I opened the _conf.php file in mysite.. And pressed the "About" page in the admin and clicked the behaviour and todo tabs.. worked fine..

Then without reloading the page, i changed the theme in _conf.php file. And then clicked the todo tab and behaviour tab. And then it didnt work..

I changed the theme back again and pressed the tabs and it works fine, note this is without reloading the page.

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.