Hola R

both, the present or the imperfect subjunctive can follow "aunque" to express a hypothetical event. You can say:

Aunque llueva después, iremos de paseo.

Aunque lloviera después, iríamos de paseo.

1. with the present subjunctive, that event seems more probable. It is as if the speaker already was suspicious about the rain because he can see a grey sky. 

Also, with the present subjunctive, the natural tense in the main clause is the future (iremos)

3. When we use aunque + imperfect subjunctive, the event becomes a lot less probable to happen than when using the present; and the tense used in the main clause is (naturally) the conditional. 

There is a link at the bottom of this lesson that takes you to the lesson on Aunque + present indicative/present subjunctive.

Saludos

Inma