Changing Ubuntu's royale purple

It seems like Ubuntu uses a lot of purple, but it turns out it only in 5 places.

• grub - i.e. the boot screens
• plymouth
• terminal background 
• desktop background
• login greeter background

Presuming you want to change the background to something dark, i.e.  you don't want to invert light and dark tones, the job is pretty easy.

Grub

Create a background image match the size to native screen resolution. Save it to /boot/grub/background.png

Edit /etc/default/grub adding

GRUB_BACKGROUND="/boot/grub/background.png"

Run sudo update-grub

Plymouth

Plymouth is the screen that shows after grub while the laptop loads.  I remove quiet from grub boot lines so I get to see the steps of the Linux boot process. I have nothing to change here, if you prefer the Ubuntu logo and loading dots its configurable and easy to test changes https://wiki.ubuntu.com/Plymouth

Terminal

Edit > Preferences > profile > Color (tab)
uncheck "use colors from the system theme"
Change only the background. Set something below #303030

Login Screen

The theme for gdm is written in CSS, files in /usr/share/gnome-shell/theme/, you may have more than one option.

Find existing backgrounds

cd /usr/share/gnome-shell/theme/
grep -r -A4 lockDialogGroup .

The #lockDialogGroup elements define the background.

To just make it black

echo '#lockDialogGroup { background: #000; }' > ubuntu.css

Desktop background

Right mouse click on the desktop and "change background".



Et voila,  No purple.

I imagine this is subject to change over time, I have done the same steps on 18.04 and 20.04. Despite the change in Ubuntu its not lost its twiddleability.