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Regarding Themes and css:
In your installation you will find a folder named themes. It contains all theme folders. The actual theme folders contain the HTML theme templates, as well as the css file.
Though note, there is also an extra css file int the actuall root dir.
Your installed modules might come with their own extending style sheets. While one would also expect these to be found in the template folder, you will find, that with some Modules you will actuall go digging in the code, to find where the settings are hidden.
I is very easy when developing a mod, especially as a beginner, to not follow the princepal of dividing layout and content.
Regarding you last question, could you explain a little more. I am not sure I understand. What exactly do you want to include? Before I give you any foolish answers...
Hope this helps so far...