The problem is that I want to change just one element or two from the default theme, but Firefox Color changes them all to the preexisting colors in its "Custom Color" tab, the moment I click on one and change it, so I should manually revert all the others to the original default.
This is very, very badly conceived. I should be able to change just the color of one single element, without the others being changed. I don't want to waist my time reverting all the others to the default, for which I should know what the defaults are in the first place, and I don't... BAD!
This is an extension made for those who want to play with it, not for those who just want to correct a few elements, without the fuss of having everything changed against their will, and then to have to painstakingly revert back all the colors they didn't want to change.

One easy solution would be that one of the preset themes in the "Preset Themes" tab, would be the the default one.
But of course that would not be a solution for those who already use a theme other than the default, and want to change just one color or two... The concept of this extension is wrong from the start...

PS: and You left out an important customization: the color of the sidebar itself. Hard to understand why...