Hi,
The tree content is added by the type "shop_category".
This class generates the content used by the javascript library in order to create the html tree.
The minus icons and the sub levels are also in the tree content. So you have to modify this type class to change the content.
Once again, the best will be to integrate this feature in HikaMarket with a sort of extra option parameter ( $options = array() ) so it will be possible to add new specific features without modifying the "display" function parameters again and again.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.