[FEAT] Allow changing of theme primary color for accessibility

[Biowulf21]

Description

Hello @sadespresso. First off, I want to thank you for sharing your work. I've been looking for a less bloated expense tracker for a while now. I came across flow and I'm loving it so far!

My problem is that the default primary color is a bit unreadable for me as I have vision impairment issues.

Being able to pick a primary color is a quick and easy way to be able to solve this problem.

I am currently working on this feature.

Additional Context

No response