QR codes in connection settings aren't recognized by some readers in dark theme

Some readers don't recognize QR codes with inverted colors. You may call it a "you" problem, but still.

Here's the responsible code:

• https://gitlab.torproject.org/tpo/applications/tor-browser/-/blob/b2ffba38205d148463a9471e866e5e9dc8d37673/browser/components/torpreferences/content/bridgeQrDialog.jsm#L30-31
• https://gitlab.torproject.org/tpo/applications/tor-browser/-/blob/b2ffba38205d148463a9471e866e5e9dc8d37673/browser/components/torpreferences/content/connectionPane.js#L605-606

Simply hard-coding black and white (or keeping the default ones) regardless of theme wouldn't help because the image is drawn directly on top of dark/light background, depending on color theme, which makes it unreadable if the background is dark. Although we could add a padding element with white background.

Or we could just wait until all readers start recognizing inverted QRs.

Similar issue in another project: https://github.com/bennyguitar/BTCDonationViewController/issues/1#issuecomment-39025558

If you agree that we should hard-code black and white, then I'd like to implement it.