Being not an official OpenWrt component... the appropriate place to ask this theme specific aspects of this question is via the developers support mechanisms ( in this case github issues )... Luckily Jerry is also 'active' on the forum so you could also ask here... That said... the theme 'config' you refer to is a separate ipk... ( which Jerry clearly explains in his github readme )
Knowing this it becomes a how to integrate a third party package question... ( i.e. a general package integration question )
so you need to handle that via your respective installation method ( buildroot-feed / router-ipk / imagebuilder-pkgrepo )
these processes are well documented
( fwiw: i'm using './files' and 'uci-defaults' to install the ipk/s during firstboot... )
ARGONTHEME="/etc/custom/luci/luci-theme-argon_2.2.5.ipk"
ARGONAPP="/etc/custom/luci/luci-app-argon_2.2.5-1.ipk"
if [ -f "$ARGONTHEME" ]; then
if ! opkg list-installed | grep -q '^luci-theme-argon'; then
opkg install $ARGONTHEME 2>/dev/null
fi
if [ -f "$ARGONAPP" ] && ! opkg list-installed | grep -q '^luci-app-argon'; then
opkg install $ARGONAPP 2>/dev/null
fi
fi