Add theming support and other changes.

[AllanWang]

The main point of this commit was to move all hardcoded colors to the color.xml However, the ones in html (and strings) are not changed, as I don't know how.

Also, I updated your gitignore to ignore gen/, .idea, and .iml files among other things. Those should not be uploaded to git and may cause errors if you pull from someone else's build.

The app built for me, but with the git changes, hopefully it will work for you as well.

Thanks!