Even if the default Theme is good, an own and custom Theme for the Panel would be for bigger Company's better and to make a Onboard Theme Customizer would it make it such simpler to customize the Panel.

Bro, it's 2020 and they still don't have it. Even FB and YT have managed to do it already. We've moved servers and thus changed from Direct Admin to Plesk and holy ****, it's ugly. It's not just that it's light mode only, but even the colors in light mode are trash. Some dead green, as if some IT guy from the 90's wearing khakis picked it. There's a 2nd option (they're spoiling us!!) that looks better on the thumbnail, but guess what... it's bugged. Top bar is white and so are the fonts, so you can't see anything, and the color on the left menu is barely different from the 1st option (even though thumbnail shows it's almost black, they don't match) :D

I agree with Mitchell. We had in previous versions a customizable block where you could change, at least 3 things. I would love to have, if nothing better, the old customization extension.

There was another idea about a Dark Mode for Plesk which has been declined but I think this wasn't fair. The Theme Extension isn't that much customisable at the moment and you can only switch between Obsidian and Onyx (if I name them correct).

In the past you were able to change individual colours to make a theme to your likings. Since this is not possible to add any customisations, I think Plesk should at the possibility to add a dark mode, since the white background is really white. Dark Mode is becoming a thing lately and I think Plesk should follow.

If we can't get more out-of-the-box themes, can we at least get information on how to customize the CSS and create our own theme?

In Plesk Obsidian there are only 2 color options available.
More color options would be much more useful like Onyx has.

User can customized the UI according to their business so reseller can provide new theme based UI to their customers