Here is one document you may find helpful.
But even this leaves much to be desired on getting you started with themeing. You do your custom CSS when you work with the themes. Here is the basics.
For your first folder call it globaltheme
Within this folder have a file called theme.xml
<description>Corporate Theme for My Company</description>
Then add directories
Add any images you want in the images directory.
Add theme.css in your styles directory
Add template overides in your template directory.
Add the css changes in the theme.css file.
When done, zip up the globaltheme directory. Then you upload it into the cloud tool if your site is hosted with jive.
Hope this helps.
You may be able to upload your theme in the section of "Customize your Site" and click "find out how to create your own theme".
You may also upload the theme direct into the app at /usr/local/jive/applications/sbs/home/themes/
Afterwards, you need to apply the theme in the admin console under themes.