How do I change the color of the sidebar when I click? Without using the source code? Is there any?

Hi there,

Most customizations in Frappe are done by building a custom app. This allows you to create persistent behavior without editing the Frappe or ERPNext source code directly.

In the app, you can use a hook to add your own custom css:
https://frappeframework.com/docs/user/en/python-api/hooks#desk

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Like what I mean is active sidenav, how can I change the color?

Right, I understand your question. It’s not possible from a setting. Things like sidebar color is controlled by css files. You need to create an app that can hold a css file.