I noticed you use both “colors” and “colours” on the dialog, that should probably be fixed.

One idea would be to have a “Copy colors from current theme” button, so that you could easily tweak a theme.

“OR”

Make the color target names easier to understand. What is “Normal” ? The coordinate system to match column title with row name isn’t very intuitive. In other words it requires studying for a few seconds. Since you only have six variables total it doesn’t seem too wasteful for screen real-estate to dedicate a more readable description of what the user is actually changing for each item (a mini preview, a complete title, etc).

Purpose: Makes fine-tuning a color selection easier if you can see the changes in real time.

Just a comment on the nature of the checkbox. It seems to be the opposite of what it should be. What I mean is that the user checks the box (usually meaning ‘enable’) in order to NOT use the custom theme controls.

Perhaps it would be more straightforward if the checkbox was the opposite, where the user checks/enables it to use the custom theme controls. For example it could read “Use custom colors rather than the theme default colors”. The controls below are disabled until the user checks the box.

The verbiage may need changing, but I hope I’m being clear.

To make things better, I would like to suggest thinking of how you could utilize gradients on it. E.g. gradient from white to pastel-pink on a pastel-pink titlebar would look a lot less intrusive than this dark as blood titlebar. Then the content area would work better as gray / white. Pink scrollbars, progress bars etc. could be ok, but not as a paint of the whole surface, especially not as this strong. The surface could on the other hand have pinkish gradients, lines etc. with a low contrast, but anything strong would ruin it very easily.