Fix Frame fg_color inside ScrollableFrame

[Yivi64]

Frames inside ScrollableFrames did not get a new top level fg_color and remained the same color as the parent. Implemented a fix by checking if the parent of a Frame is a ScrollableFrame and changing the fg_color to be like any Frame inside another Frame.

See here for the before:

And After:

[FedericoSpada]

Hi,
I don't like your solution: the problem is not due to how you wrote it, but the fact that the existing code made you write it like that (for example, you can't import CTkScrollableFrame directly because it would create a recursive import condition).
I think we should work to change CTkScrollableFrame to inherit directly from CTkFrame instead of tkinter.Frame.
In that case, the initial check if isinstance(self.master, CTkFrame) would also be true for CTkScrollableFrame. Moreover, we would also get the same effect if a CTkScrollableFrame is placed inside another CTkScrollableFrame.