There was lately clean up for dashboard styles. Many !important statements was removed. Why those still exist, I don't know. But with latest version, for me only the h1 ... h4 rules are affected. Any other like margins and stuff are free to use as you like.
But let's see what @dceejay says about the .nr-dashboard-templateh1 ... h4
I just reinstalled nodered couple days ago, so I guess I have the latest version. But as u can see on my screenshot it affects the <p> tag aswell.. wich is not so nice. I can try to put into a <span> instead of h1 or p
Don't guess the versions, you can look out of them. But yes, for sure the heading and paragraph tags today fight back hard. Use div - span combinations and dedicated classes and you have freedom to do anything.
Any user created something on top of existence of that will have to change something. And as it is "build your stuff" node, it is impossible to predict the impact. If it was my call to change it today, it would be no go. Or if, then with long therm announcement for breaking things.