Credits go to @kmh for creating this style. It comes in both a light and dark version (check the links for usage instructions).
If you don’t want to fiddle with code and stuff, loma.ml has already installed it server-wide*, and should be available straight away for new accounts (If not, go to Settings>Display>General Theme Settings and change it from there. Or follow the guide in any of the links above).
Edit: loma seems to require invites. But, while in german, joinfriendica.de allows you to join without an invite. It also has quite a bunch of add-ons too like bluesky, tumblr, ifttt, markdown, langfilter (filters the languages in posts/comments) and curweather (shows the current weather).
Anybody who gives a rat’s ass about grandma wants to see her hanging out in places that aren’t actively hostile to her, and will at least consider switching.
If they insist on keeping her in places that want to see her come to a bad end just to make a buck, then we can safely stop calling or thinking of those people as friends.