This may not be what you want, maybe you have other plans for the highlight, but instead of creating it in your row template you can just pass a highlight template to ZO_ScrollList_EnableSelection(...)
and pick a different highlight texture:
Lua Code:
local selectTemplate = "ZO_ThinListHighlight"
ZO_ScrollList_EnableSelection(scrollList, selectTemplate, selectCallback)
ZO_ThinListHighlight
(You probably wont see the black on the right hand side your rows aren't that wide)
If you don't need a callback & just want a highlight you can just leave the selectCallback nil & it will just highlight/unhighlight the rows for you when they are clicked.