If a warrant has been issued on me for theft will the police actively pursue it?

It really depends on the jurisidiction the warrant is from and the facts of the underlying case. Often the police do not actively seek out people with warrants but instead wait until they are contacted by police for some otehr reason, then the warrant is located and the person arrested.  However, it can be an advantage knowing the warrant is outstanding, especially if you know what bond amount ($$) was set on the warrant, as then you can turn yourself in, at a time convenient for you, and be prepared to post the bond.