If you haven't noticed 90% of people are running windows which is quite frankly ugly by default. Further it was pretty hard to get people to give up fisher price windows xp. It's probably accurate to say that most people aren't terribly concerned with aesthetics as much as functionality.

That said steps to achieve a relatively consistent look.

Open your configuration app and configure your gtk theme to something that looks nice. If you don't have one you can install lxappearance which is from lxde and has minimal deps so you can easily install it in a minimal environment.

Install qt4-config if you have any old qt4 apps. Set it to use gtk+ style for qt4 aps.

If you use newer qt apps install qt5ct and select gtk style.

In 5 minutes you will have a relatively consistent look.

A few notes.

- Its increasingly likely you have zero qt4 apps and could probably skip that part and not care

- qt5ct actually lets you set a custom stylesheet and or colors if you want more manual control but this is hardly required.

- firefox is really stupid about drawing website elements with dark text on a dark background when run with a dark theme. You can run individual gtk apps with a different theme like so

env GTK_THEME=Adwaita:light firefox

you can copy your firefox desktop file to ~/.local/share/applications and edit the command to ensure that this works properly automatically.

Chrome somehow figures out that a dark theme doesn't mean you want to draw webpages differently so I consider this a firefox bug that mozilla wontfix.

In a perfect world someone would create a single useful gui to configure qt4 plasma gtk2 gtk3 apps which only had settings that would effect all of the above consistently and required no tweaking.

In reality land most people don't care including those shipping distros can probably ship with or arrange a reasonable look with minimal fuss.