add background to png export

[sornas]

right now it is transparent which does not look good at least in discord

[l0f3n]

The button is kinda cool, but I'm down to just let users take screenshot at that point. Then they can choose if they just want the graph (with the background) or the leaderboard as well. I might just nuke the export button, what do you think? Right now I don't think its useful and we would probably add a similar button later for #23 later anyway

[BeeSneeze]

I think maybe adding export options would be a good idea, since sometimes you want just the graph, sometimes the leaderboard, sometimes both etc. I'm gonna give it a bit of a think during the week