The OpenBox Configurator plugin to LXAppearance was released some days ago. Grab the file from sourceforge.

lxappearance-obconf-0.2.0.tar.gz, sha1: c607bf93b7941d21f9b29e6d6429b48fe2f7e388

Changelog
75f08b6 – Fix #3465609 – LXappearance can’t install an obt theme since openbox 3.5.
3cd5475 – Fix broken theme preview for openbox 3.5.
aecce51 – Support Inactive OSD font introduced in openbox 3.5.
b56977a – fix error handling when saving the config file in tree_apply()
a2c6e85 – fix the incomplete rename started with 966cb46
966cb46 – allign with obconf upstream code
783e544 – fixed trivial error
43b054e – Load the rc.xml config file given on –config-file the same as Openbox (use the whole path given).
f47573d – it should load the config file now
8f40b0c – make it build with openbox 3.5