Download the one you wish to add and open it in a text editor on your computer. If you are on Windows Notepad++ is the best one.
Hit ctrl+a to highlight everything, and ctrl+c to copy it.
Go into the admin panel on mybb and go to the default theme. Go to the "Add Stylesheet" tab. For filename put the name of the css your going to copy in. An example would be color_black.css for the black one. Change the radio button for the content source to "write my own content." You should now see an editor appear. click in the text area of the editor and hit ctrl+v to paste in the content. Then hit "Save Stylesheet"
This will re-add the stylesheet to your theme. Unfortunately there is no way to just import the file...
That would probably be a good feature suggestion.
Edit: Oh, and make sure that under the "Attached To" section you hit "Specific Color" and tell it which color that one belongs to.