a border for the board would look nice

First: this site is absolutely amazing, great job!!

but as the title says, a border for the 2D board would make it look much nicer - just a suggestion from me

This is great idea and i agree.

Here's a simple userstyle you can use: userstyles.org/styles/140164/lichess-org-border-around-board. You can easily adjust the color by playing some with the rgb values.

like that whoud be amazing, unfortunately I dont know how to make it