You may have difficulty in discovering whether your play is thirty minutes or not. The most basic convention is 1 Page = 1 Minute, but this is hardly ever the case. You will have to read, re-read and possibly perform the play with friends to gain a better grasp of time. To a marker, it is dead obvious which plays are in the time frame.
Thematically, there is no restriction. Download the Scripts in the 2005 BOS Showcase, in the main body of the English Extension 2 forum, and you will see themes from parody to politics to reality television to death.
Best of luck