Firstly, you have not said what platform or what version of Grafana you are using so it’s going to be difficult for people to point you in the right direction.
If you don’t want to change system files you could try the Boom Theme plugin. It’s a bit old/clunky but it seems to still work with Grafana v7 and enables various things including custom CSS. I use it for some CSS tweaks on dashboards. You add a “hidden” boom theme panel to your dashboard. I think you have to do it on every panel though, not sure if there is a way to do changes globally.