Welcome to the forums!
There is no library as such, its all in the stylesheet.
Stylesheets can be edited in an external text editor, such as notepad++, Flare's internal text editor, or the stylesheet editor (double click the stylesheet in the content explorer). So if you want to add stuff manually you can. I'll do that sometimes when creating a new project so I have a baseline of styles to work from.
I prefer advanced view.
If you show all styles in advanced view (and simplified view come to that) you will see the Madcap items that get converted during build.
You can add your own scripts but I've not had to play around with that so you will be better off waiting for someone like Dave Lee to chime in...
HTH