I think most of us here are more concerned about getting the job done efficiently then on having eye pleasing screen. I rely more on my ears than eyes when working with Ardour.
But since you mentioned Fluxbox, I have been using it for some ten or so years when working with the audio and I have some good things to say about it.
It boots 3x faster then KDE on the same hardware, it uses around 200mb of RAM on startup, it is as responsive as can get, it has a ton of options and possibilities in terms of keyboard shortcuts and they are easy to add /edit. This helps a lot when moving a dozen or so plugin windows to or from virtual desktops, minimizing, maximizing, toggling… It also has possibility to remember size, placement, layer, decoration… for each app. I find it easier to have the main Ardour window always maximized, without decorations, and layered at the bottom so I don’t have to deal with plugins “disappearing” as soon as I move the playhead. I like to have them on top and minimize them when I want to. In theory, one can add these “window rules” in KDE, too, but I didn’t have much luck with that… apparently KDE has problems discerning Ardour main window from Ardour plugin windows (no such problem with Fluxbox)…besides, KDE swaps way too eagerly.
As for the look, default Fluxbox looks hardly usable, but with little effort it can be “riced” to look at least bearable, all you need to do is to edit a couple of text files. As for themes, there are plenty of those to choose from tenr.de.
And now some pictures:
Fluxbox desktop with native menu and dmenu launcher:
Default Ardour colour scheme has always looked just fine to me, Ardour6 looks even better, so no need to change anything in that field. The most important thing is to use as much of the screen area as possible so I made my toolbar as small as 14px, it is only needed to bring up plugins and minimize them when no longer needed: