Hi Gian,
Thanks for contacting Syncfusion Support,
We have checked the reported query with our SfGridSplitter control from our end. If you need to change the UI of horizontal splitter up and down collapse button, use UpButtonTemplate and DownButtonTemplate properties. If you want to change the UI of vertical splitter left and right collapse button, use LeftButtonTemplate and RightButtonTemplate Please check the sample and the documentation link for your reference.
Please let us know if you need any further assistance on this. We will be glad to assist you.
Regards,
Bharathi R