Why should I care?
Thanks to GPLv3 if someone wants to distribute this theme with or without changes he must give the same rights he got from me to the people he distributes the changes helping Free Software growth.
Does it mean I cannot sell changed/unchanged copies of this theme?
Of course not. You can sell copies of this software with or without modification as long as you grant your clients the rights of GPLv3 to edit and further distribute your copies.
Should I distribute the source code if it is modified to my users if I use this software as a service?
No, you can do it if you want and would be great you do so, but it is not enforced.