1) From the GPL faq :
Does the GPL require that source code of modified versions be posted
to the public?
The GPL does not require you to release your modified version, or
any part of it. You are free to make modifications and use them
privately, without ever releasing them. This applies to organizations
(including companies), too; an organization can make a modified
version and use it internally without ever releasing it outside the
But if you release the modified version to the public in some way,
the GPL requires you to make the modified source code available to the
program's users, under the GPL.
Thus, the GPL gives permission to release the modified program in
certain ways, and not in other ways; but the decision of whether to
release it is up to you.
In some sense, by using your modified theme for your webapp, I would consider that you are releasing your theme for something else than internal use, so you need to release your modified source code, at least to the person using the app.
2) GPL never forbid any kind of charging for the use of your app, so you have the right to make money of it, as long as you comply with the terms of the license.