You can find the default theme settings down in the sidebar block of the editor page. The size and colours of the control panels and the colour behavior of popups can be changed in this block.
You can set here the deafult theme size from XS to XL.
There is an option to choose primary and secondary colour of the theme.
Popup window colour
You can set here the colour behavior of the popup window: default or control.
Learn more about theme settings!