You can go to studio and locate your default theme that you've set  sample protean to customize existing go to protean and click styles

you need to copy or add new mix from original styles name it to whatever you like

hover on image attachment if you cant see, then right click and view image below.

In new added mix or design you can now edit colors and stuff

In splash page click this icon

click splash then you can edit this part

If you need more help pm me. We can design. Have a nice day.