Restores Tk-compatible bg and background aliases for CTk.configure

[ajkessel]

I've started running this code through updated unit tests based on my https://github.com/ajkessel/Custom2kinter/tree/m1-test-foundation branch. I will eventually propose those branch as a PR, but first I wanted to get some fixes through that the unit tests have surfaced.

This PR restores Tk-compatible bg and background aliases for CTk.configure(). These now route through the existing fg_color handling path, so cget("fg_color"), cget("bg"), appearance-mode color application, and child color propagation stay consistent.

[FedericoSpada]

Why would you need this?
bg and background are not valid arguments (they are not present in ValidTkArgs). In CustomTkinter, "fg_color" is the chosen name to refer to the primary color of all widgets. You also risk confusing it with "bg_color".
Moreover, other widgets also have the "bg"/"background" attribute, but it has been renamed "fg_color".

cget("bg") allows you to retrieve the currently displayed color, taking appearance_mode into account, but this is more a side effect of the implementation than an intended feature. configure(bg=...) should not be used and should result in a type-check failure.

When I moved the constructor arguments to dedicated TypedDict classes, I looked for a way to specify that the cget() method can accept only keys of the TypedDict (something similar to Unpack, but for the keys only). Unfortunately, I didn't find a good solution that didn't require maintaining a separate Literal[...] type with all possible values.

[ajkessel]

OK, that makes sense. I treated this as a compatibility regression because the legacy unit test from customtkinter used configure(bg=...) / configure(background=...), but your explanation clarifies that fg_color is the intended CustomTkinter API and that cget("bg") working is just an implementation detail.

I agree that adding bg / background to configure() would blur the distinction between fg_color and bg_color, and would also weaken the TypedDict argument checking.

I’ll close this PR shortly and update the pytest updates I'm working through to stop asserting configure(bg=...) support. The test should use configure(fg_color=...) for the supported public API, and avoid treating cget("bg") as a configurable round-trip contract.