Tweak default colors (softer white/black)

[acestronautical]

People may uninstall before they try to change the defaults. #ffffff and #000000 are too harsh for most users.

[m-khvoinitsky]

In general, I agree with rationale you provided in #222. While I personally prefer "hard" dark colors, I admit that the majority would prefer more softer colors. However, current algorithm do not perform well enough with softer colors, for example, if add-on decides to preserve background color hue, it selects the darkest luma possible which results in patchy background. You can set background color to relatively bright dark color like #2A2A2E and look at github issues page, for example. To resolve this, I experimented with YCbCr color space and enforcing default (configured) color's luma everywhere. This performed very well almost everywhere except for where it didn't: there are places where luma heterogeneity is important.
You can try new_color_processing branch where this behavior is implemented and see, where it works good and where it works bad. In particular, if colors are just part of design, it works good. But if colors are part of some meaning like table cell background colors in MDN compatibility tables it works not very good.
Second problem: the values itself. I would prefer to set them to the values of Firefox in-content UI, in particular, this ones. This would make page colors to match Firefox pages like about:preferences, about:addons and others.
A couple of barely related issues which I however would also wanted to implement before changing default color scheme in order to decrease inconvenience: #182, #4 and probably ability to export\import preferences to a file.