fix ui flashing on reloading and fast scrollong

[light-and-ray]

Description

Fixes white UI flash after page reloading/loading and fast scrolling. It happens because css from files can't be loaded so quickly. Disadvantages of patch - black flashing on white theme. But I think it's less critical for eyes, and white themes are less popular

Screenshots/videos:

Before:

Screenrecorder-2024-07-05-18-54-20-924.mp4

Screenrecorder-2024-07-05-19-18-47-491.mp4

After:

Screenrecorder-2024-07-05-19-16-07-790.mp4

Screenrecorder-2024-07-05-19-17-47-417.mp4

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[AUTOMATIC1111]

White is the default and it's also what I use.

[light-and-ray]

I think it's less critical for eyes, and white themes are less popular

[w-e-w]

@AUTOMATIC1111

I made some improvements

it will now apply different colors for light and dark theme

note based on system of browser preference
the ?__theme=light ?__theme=dark doesn't work

it resolves the color from the gradio theme background_fill_primary so every one should be happy with the result
it should not flicker "at all" as long as the theme is using background_fill_primary as the background color (which it should unless some strange thing is going on)

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I've introduced a new function modules.shared_gradio_themes.resolve_var this should be able to reslove a gradio them name such as background_fill_primaryto it's color in some cases it would resolve as a function like (likergb(), linear-gradient()` ...)

but I don't know if there's too much I can do about that without overcomplicating things

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personally the flicker is driving me crazy

• this PR compliments ReloadUI backgroundColor --background-fill-primary #15864