Use photon colors consistently

[ariasuni]

Also fix about:addons icon not colored correctly in dark theme and improve a bit new tab button margin and centering.

[Keith94]

Hey @ariasuni
I found an issue with our icon using the wrong colors. If you have time, check it out. 😃

[ariasuni]

This bug is already present in the current version of the extension and it seems it can’t be easily fixed programmatically. The icon displayed in the header bar and in the list is the same, but with the default dark theme of Firefox, one’s white and the other dark, and we can’t really know what’s the currently used color for each.

The best fix I can see is to make an icon that works both on light and dark backgrounds, but’s that’s not the in the scope of this PR. Would you mind opening another issue for that? I’ll then copy/paste my comment there.

[Keith94]

Thanks. Here it is: #392