Possible to have different themes for different users?

[Efficient_Good_5784]

As far as the client side CSS goes, does that mean there is essentially no way to have CSS for individual users? I was under the impression that each user could have their own custom CSS, but I will re-evaluate if that is not the case.

There is. It's just left to the users themselves instead of the admin to control.

The admin can apply a server-wide CSS theme.
Users can disable the admin-created CSS theme and add their own. Note that users don't have to disable the admin CSS theme, they can add onto the admin CSS theme too if they want.

The setting for client-side CSS is found at: 

Client Settings > Display > "Custom CSS code"

The client CSS code states: "Custom CSS code for styling which applies to this client only. You may want to disable server custom CSS code."

When I stated it's not available on some apps, it's not available on clients that use their own interface.
Basically, if the client uses the web interface (browser, Android Phone app, etc), it should have an option for client-side CSS code.

Android TV doesn't support any CSS code (both admin or client-side) because it's not using the Jellyfin Web interface.

Finally, some advice for customizing your server with CSS. A browser's dev tools will be your friend here. F12 and "crtl+shift+c" to select the CSS element of an item on screen (at least on Chrome based browsers) will be what you'll be using most of the time. You just have to copy the name of the items that appear in the dev console and apply your own modifications (such as changing the color with rgba() declarations). You can also have fun with this and modify any site you find online. Any modification done in the dev tools will be reset upon your next reload.

You know your girlfriend the best here. If you think she can handle doing her own customizations to her client, just let her know where to add the CSS code. If not, just do it on your own and share the code with her.