Hi Anhar,
Thank you for reaching out to Mixpanel's Support -- I'm Varsha and I'm happy to help with this!
I hate to be the bearer of bad news, but Chart Customization is currently only available for Enterprise plans as it is a fairly new feature. I took a look at the organization you are part of and it looks like you are currently on a Free plan.
In the meantime, I see how customizing colors and themes for reports would be useful for all of our customers so I've gone ahead and filed a feature request internally. This will allow the Product Team to evaluate the development of this feature for future releases of Mixpanel.
Sorry to be the bearer of bad news here. Let me know if you have any additional questions!
Best,
Varsha