YaST doesn't have support for external icon themes without making changes to
links inside theme directory. Data inside of
library/general/src/modules/Directory.rb in case of Wizard themes always has
higher priority over system theme. In case of icons, removing icon symlink
doesn't have effect on icon theme and falls back to hicolor, as opposed to
system theme which affects YaST when there is no theme with higher priority in
/usr/share/YaST2/theme/current/wizard directory.

Icon module in library/general/src/modules/Icon.rb doesn't take any other
option than .png into account (it's not the only lib that does that). It would
be nice to at least have additional support for SVG, even though it's optional
in freedesktop standard
(https://specifications.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#definitions)
(but I don't see you supporting .xpm, so at least support .svg which is at
least somewhat popular on desktop).

I couldn't find mentions about pattern icons and package groups icons in
software manager, but those don't even want to change with icon theme symlink
(/usr/share/YaST2/theme/current/icons) changed to something different. It would
be awesome if those followed the same spec. For now from users point of view
they seem to be hardcoded and impossible to be changed to anything different.