The configuration of dvtm 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 dvtm-$version/config.def.h to
dvtm's port directory, and edit to suit your preferences. Then,
when you call pkgmk in the dvtm 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 dvtm 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 dvtm is to create a custom and
(preferably) private port that contains your custom config and also
might include patches for added features.