Change RGB colors to symbols in Themes

The [themes file](https://github.com/SciRuby/rubyplot/blob/integration/lib/rubyplot/themes.rb) contains various color names that are current stored simply as RGB values. These should be changed to symbols denoting names of the colors and the relevant entries should be made in [colors.rb](https://github.com/SciRuby/rubyplot/blob/integration/lib/rubyplot/color.rb).