Let's make our own custom theme. We'll do it in Online/Enterprise Edition first, but I'll comment on Developer Edition at the end.
I am using the JewelryBox Template that defaults to the Violet theme. If I just want to use a different theme, I can change it by hitting the paint palette on the left side of the screen and picking a different choice. But none of the default themes have my color of blue, so I'll go to the bottom of the screen and pick Customize.
It brings up this panel:
From top to bottom the options available to you are:
