Onion pattern on about:torconnect needs a dark theme asset

Between Tor Browser 13.0 and 13.5, we simplified the onion pattern present at the foot of about:tor and about:torconnect. In the case of the latter screen, an asset was supplied for light theme, but not dark theme.

Looping in @henry for implementation (when ready). See !1025 (comment 3029510) for details.