How to Install scripts? (Click here)
Since this script https://greasyfork.org/en/scripts/2299-myanimelist-custom-css-override
doesn't work I made one myself.
To add/update/change the default CSS style that will be used on any anime and manga lists on MAL:
1 Install the script.
2 Open any anime or manga list. (The whole page will be broken and white at this point, till you follow step 3.)
3 Click on the "Use this List Style!" button on the tampermonkey extension menu. (This will show you what's the actual user css style, and will give you the option to make the displayed style as the default override style or not.)
4 Done! That anime or manga page CSS styles will become the default css style that will be used on any anime and manga lists on MAL!*You can also follow the steps 2 and 3 to use any custom CSS that another user made for their own anime/manga lists on MAL!