UI dark color scheme implementation

[bgptr]

This PR implements Dark Mode: Master Issue (#2089)
Sub issues:
- Dark Mode: Navigation (#2090)
- Dark Mode: General layout (#2091)
- Dark Mode: Send view (#2092)
- UI dark color scheme. In-app views (#2122)
- UI dark color scheme. Launcher views (#2123)

[vctt94]

Thanks for this PR.

Lint is failing:

ERROR in ./app/style/Themes.less (./node_modules/css-loader??ref--8-1!./node_modules/less-loader/dist/cjs.js??ref--8-2!./app/style/Themes.less)
    Module not found: Error: Can't resolve '../style/icons/sortByDark.svg' in '/home/travis/build/decred/decrediton/app/style'
     @ ./app/style/Themes.less (./node_modules/css-loader??ref--8-1!./node_modules/less-loader/dist/cjs.js??ref--8-2!./app/style/Themes.less) 7:11003-11043
Child mini-css-extract-plugin node_modules/css-loader/index.js??ref--8-1!node_modules/less-loader/dist/cjs.js??ref--8-2!app/style/Tooltip.less:

[bgptr]

Thanks! I try to figure it out why

[vctt94]

Thanks for this PR. It looks pretty good overall.

I have just some small comments.

The text to connect to the daemon in advanced mode still black

I do not like the icon's hover on the sidebar, because they barely can be seen. But this needed to be discussed with the design guys, not about this PR.

hover example:

I like icons being variables now, they are more centralized, so it will be easier to find them.

[bgptr]

Thanks for the comments.
For the first issue, I could not find any guidance in the design spec, but I did something now:

@linnutee, please review this change or any other implementation issue in this PR.

I did not like the icon's hover on the sidebar neither, but they are animated icons. If it will be re-rendered, I am willing to fix it. @kyleFirethought, could you help me?

[matheusd]

We'll need to iterate on the dark theme, but indeed seems like an improvement.