The configuration of sxiv is done by creating a custom config.h and (re)compiling the source code. To create a custom config.h, unpack the tarball, copy the file sxiv-$version/config.def.h to sxiv's port directory, and edit to suit your preferences. Then, when you call pkgmk in the sxiv port directory, the custom config.h will be detected and copied into the source before compilation. I recommend that you save a copy your custom config.h some where other than the sxiv port directory, so that your changes won't get nuked when updating ports or performing a prtsweep or prtwash (see opt/prt-utils for more information about those two). Another solution for customizing sxiv is to create a custom and (preferably) private port that contains your custom config and also might include patches for added features.