Update Icon colours in address bar/doorhanger

[pdehaan]

Via @e-pang in mozilla/blurts-server#622

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@nhnt11 Are we able to have a different colour for dark theme?
Also, I think the colour needs to be updated for the default theme as well - it looks a little dark (could be wrong though!)

Here's what the colours should be:

Light Theme:
grey 90 (#0c0c0d)
60% opacity

Dark Theme:
grey 10 (#f9f9fa)
60% opacity