Revert design change on Manage Themes dialog

[Jan Six]

Sometimes in 2.0 we introduced a design change to the Manage themes dialog that made the whole thing a bit less user friendly. Let's change this back:

• Move the input to the left of the theme name input (this will fail for long theme names or small window sizes, but we can tackle this in a more elaborate UX fix when we align with Studio)
• Make the button a Button with variant secondary
• Add a dropdown prop (arrow down) when there's other groups to pick from. If no other groups exist, dont add the dropdown arrow.