I’ve tried to hack into the css but Miro probably uses some canvas.
So, probably the script has to be modified with a browser plugin.

You don’t have to use a frame, you could just create a shape with white background color.

Hi @Kostas, @mlanders​​, @Barnabas Szabolcs, and @Henrik Ståhl! Exciting news, I hope! You can now choose from nine new board background colors – including white from your board settings menu. Check out the board background colors announcement and let me know how its working for you.

@Kristin On the 6th of Dec on 2020 I asked for a full free colouring of the background:

https://community.miro.com/ideas/dark-mode-night-mode-874/index2.html?postid=2237#post2237

and now you are delivering a smalll vew colouring with nine colours … 

while you celebrate this competitors offering a lot more …….

Why not go the complete user-wish-way

till it meets all needs of the users ...