Reduce white flickering while in dark mode

[PalmerAL]

• Removes a workaround for Windows that shouldn't be necessary any more (because the issue was fixed in fix: BrowserView background color in webContents electron/electron#33435). Now when we hide the browserView, we should get a black flicker instead of a white one.
• Waits until the page actually starts loading to set the browserView background. Now if you open a new tab and navigate to a site that has a dark theme, it should transition directly from dark browser background -> dark page in most cases, without having a white flash in between.
  • We still have to set the background to white eventually, because some sites don't set their own background color and assume the page will be white.

I'm still seeing some remaining white flickering with this PR, however; I think it's a result of something Electron is doing that I haven't figured out.