Everything will be great if its available for user to change, but then guess what: you will end up with a product with a million configurations that you need to change, and you will be scared and confused just looking at the setting.
Beauty is in simplicity.
and If you want to change the theme of your installation, you can easily create an app and put you CSS files in hooks, and you will be good to go. Check similar approaches here
I might agree with you but this case if not a matter of beauty … but that the users/retails sales have to work on environments that due to the Sun the screen is too light + the white on the background …
By adding few tweaks (HSE perspective) * you are protecting your users from have headache or eye-ache due to brightness or colors.
I have been reading those approaches and i have even tried one but the problem is that those are new Themes that you still need to install as an APP and for you to switch from one Theme (custom) to the Standard ERPNext you have to uninstall … which on production Servers or heavy users is not the best approach.
For now, Frappe does not support theming out of the box. Your only option for now its to put you CSS file in the hook of custom app, or edit the core directly which highly discouraged and will break updates