Ok this is the code in my Vert UI:
Code:
<InvSlot item="HBInvSlotA">
<ScreenID>HBInvSlotA</ScreenID>
<RelativePosition>true</RelativePosition>
−
<Size>
<CX>34</CX>
<CY>34</CY>
</Size>
<Background>A_InvPrimary</Background>
<EQType>inventory/Equip 13</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlotB">
<ScreenID>HBInvSlotB</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_InvSecondary</Background>
<EQType>inventory/Equip 14</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlotC">
<ScreenID>HBInvSlotC</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_InvRange</Background>
<EQType>inventory/Equip 11</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlotD">
<ScreenID>HBInvSlotD</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_InvAmmo</Background>
<EQType>inventory/Equip 22</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot1">
<ScreenID>HBInvSlot1</ScreenID>
<RelativePosition>true</RelativePosition>
−
<Size>
<CX>34</CX>
<CY>34</CY>
</Size>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 23</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot2">
<ScreenID>HBInvSlot2</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 24</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot3">
<ScreenID>HBInvSlot3</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 25</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot4">
<ScreenID>HBInvSlot4</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 26</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot5">
<ScreenID>HBInvSlot5</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 27</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot6">
<ScreenID>HBInvSlot6</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 28</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot7">
<ScreenID>HBInvSlot7</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 29</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot8">
<ScreenID>HBInvSlot8</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 30</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot9">
<ScreenID>HBInvSlot9</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 31</EQType>
</InvSlot>
−
<InvSlot item="HBInvSlot10">
<ScreenID>HBInvSlot10</ScreenID>
<RelativePosition>true</RelativePosition>
<Background>A_RecessedBox</Background>
<EQType>inventory/Equip 32</EQType>
</InvSlot>
The code may look different to yours due to the fact that I use tilelayout boxes for my inventory slots.
The part highlighted in blue is the background picture of the slot, the red is the slot type. So EQ type inventory/Equip 13 is your primary, and would use the A_InvPrimary background.
All of the backgrounds and their corresponding EQ Types are listed in the EQUI_Inventory xml file.