Closes #100 which I opened few months ago.
Today, I gave it a go and was able to add the options in the theme tab.

I added the options to set the base _color, accent_color and contrast.

Problems/Stuff I couldn't figure out

• I couldn't figure out how to change the order of Custom Theme Guide Button to show it at the end (without breaking anything).
  
• Change theme_preset automatically to Custom if any theme property is changed (You have to manually set the theme_preset to Custom in order to get the custom theme working)
• Show the theme properties only when the theme_preset is set to Custom