Execute filter in theme as last

Yes set priority to something like 999 or something really high. The default priority is 10, and you currently have yours at 1 and therefore will occur first.

The priority is explained in the codex: http://codex.wordpress.org/Function_Reference/add_filter notably:

$priority
(integer) (optional) Used to specify the order in which the functions associated with a particular action are executed. Lower
numbers correspond with earlier execution, and functions with the same
priority are executed in the order in which they were added to the
filter.

Default: 10