ReloadUI backgroundColor --background-fill-primary

[w-e-w]

Description

during ReloadUI the webpage will true white regardless of dark or light mode
it's blindly annoying
this PR sets the backgroud color of the HTML body to the color defined by the theme --background-fill-primary

the body background color will be automatically removed by gradio once the page reloads
so no impact to anything else

note this solution is not a perfect soultion as during reload the page might still flash white for a brief period
but it is still better than constantly white during reload

Screenshots/videos:

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

[Turkinolith]

+1 in support of this!