The configuration of surf 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 surf-$version/config.def.h to
surf's port directory, and edit to suit your preferences. Then,
when you call pkgmk in the surf 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 surf 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 surf is to create a custom and
(preferably) private port that contains your custom config and also
might include patches, such as from https://suckless.org/patches/.