Control Panel > Styles > Edit or Add a Style > [SELECT YOUR STYLE]
Add to this to the EXTRAS section if you've heavily customized your style:
Code
table{border-collapse:separate;border-spacing:1px;}

-OR-

ADVISED:
It looks like you are using a 15 year old stock style (from 2004 / 2006). Learn more about updating to the recent version in the UBB.threads Style Packs thread:
https://www.ubbcentral.com/forums/ubbthreads.php/topics/261503/stock-ubb-threads-style-packs

"UBB70 - Light" from the UBBT70 STYLE PACK
[Linked Image]

current developer of UBB.threads php forum software
current release: UBB.threads 8.0.1 // wip: UBB.threads 8.1.0
isaac @ id242.com // my forum @ CelicaHobby.com