1) Dal faq GPL:
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
organization.
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 un certo senso, utilizzando il tema modificato per la tua webapp, ritengo che stai pubblicando il tuo tema per qualcosa di diverso dall'uso interno, quindi devi rilasciare il codice sorgente modificato, almeno per la persona che usa l'app .
2) GPL non vieta mai alcun tipo di addebito per l'uso della vostra app, quindi avete il diritto di fare soldi, purché rispettiate i termini della licenza.