You can find the default theme settings down in the sidebar block of the editor page. The size and colors of the control panels and the color 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 color of the theme.
Popup window color
You can set here the color behavior of the popup window: default or control.
Learn more about theme settings!