about:tor colour flash when opened as a new tab
At the moment, if your new tab is set to about:tor (default), then you will get a flash of white/black-ish colour when opening a new tab. This is because we render the browser/Letterboxing background before the page loads.
I think Mozilla works around this for their own about:newtab page by setting --tabpanel-background-color to match the about:newtab page background, depending on the theme.
We could try setting --tabpanel-background-color to the about:tor background colour. But it might be complicated to avoid interference with the Letterboxing background colour.
We can figure this out after the Letterboxing colour is decided in tpo/ux/design#204.