Changes to override option proposal

[fernandotellado]

I have two proposals to 'Override theme' option:

1. Change the button to 'Override theme' when this option is selected. Now the button is the same that with the other options: Export theme, and the action is really different.
2. Add a Warning when the option 'Override theme' gets selected to inform the user cristal clear that when click the button the active theme will be overridden, not exported

Thanks!