Customization of ddefault Animation Player styles

[nvborisenko]

Description
I want to customize Animation Player while it is loading the animation. For now it looks the default values for background-color (at least) are hardcoded.

motion-canvas/packages/player/src/styles.scss

Line 28 in 044c9ac

background-color: rgba(0, 0, 0, 0.54);

Animation Player manipulates HTML Elements in shadow root, what is challenging to override some CSS properties.

Proposed solution
Probably, if HTML elements in the shadow root have part attributes, then we can override the styles.

Considered alternatives
I am newbie, don't listen to me.

Additional context
I want to make Animation Player fully invisible while it is loading the scene.

[nvborisenko]

@aarthificial do you think of the proposed solution is viable? If yes, I can propose a PR.