Hi alex-b, thanks for posting on the QuickBooks Community

There isn't currently an option for dark mode in QuickBooks Online at the moment.

I agree that this would be a great feature to have and recommend submitting your idea on our new QuickBooks Idea Exchange board. We'll be reviewing all posts submitted here, and if it gets to over 50 votes, we will review it with our Product team. Thank you. 

Thanks Georgia, I've done as you suggested, and you can vote for it here .

Great, thank for your contribution, @alex-b!