Was this request for me? Now that you have 7 themes, hundreds of color combinations, you still need to set a custom color for the background? I can explain why that will never be an option in the X3 main settings:
It is poor product design implementation. If you set a custom background color, it will affect all elements overlaying the background, which have been set relative to the original background color. We use SASS
to create carefully crafted skins where hundreds of elements are synchronized to display in harmony and contrast with the background. Changing the background color, and you will end up with elements that don't match and could even break.
This can already be done with custom CSS, which is made available to you in X3. We aim to offer the ultimate website in X3, and we do not wish to complicate this ambition by translating settings into a CSS, which is superior for this task.
If we make everything into settings, we will end up with a website builder like Wix, where the quality of the application is sacrificed.