Write your own bespoke CSS to style DevTools. Unlike installing themes, you can write custom rules to style any element on the fly.
1. Visit chrome://flags and enable "Developer Tools experiments"
2. Restart Chrome
3. Open DevTools Settings (F12, then F1)
4. Under Experiments, enable "Allow custom UI themes"
You'll see a new panel titled "Tailor" where you can add custom CSS rules to your liking. Click the Tailor button or hit Ctrl + S, then reopen DevTools. You can find more elements to target by viewing the source code of DevTools:
1. Open DevTools (Ctrl + Shift + I)
2. Undock Devtools (Ctrl + Shift + D)
3. Open DevTools for the already open DevTools (Ctrl + Shift + I) and examine the Elements panel