It turns out that my theme does not support 4,3 and is breaking a whole lot of features on my site. Even the drop down at the footer and the option to change the default theme rhtough the Admin CP.

How can i reset to the default theme? should i delete all themes and is there a way to upload / get the default theme?

ACP > Customizations > Themes > Add Theme > Easy Mode > Save. That should give you the base default 4.3 theme.