Welcome Distorted Humor! White Box (WB) is great fun, I've played or run it quite a bit. The key with WB is to realize it is a game framework, lots of specifics are left out for you to fill in. It relies on referee rulings, not rules. A lot of time the rulings will be on-the-fly, so it plays very fast. You may also be interested in 'White Box: Fantastic Medieval Adventure Game', which is based on v3 of the WB rules with a few added bits to make it more complete and to fix errata. There are lots of discussions on that game as it developed here.
If you want more of a B/X feel to the game, I recently came up with race-as-class tables: https://smolderingwizard.com/2017/07/03 ... white-box/
. You mention a thief which you can steal straight from B/X, the Greyhawk OD&D supplement, the White Box Omnibus or Delving Deeper, or create your own. Personally I prefer the simplified d6-based thief skill checks from the Omnibus and Delving Deeper, I think they are more in line with how WB plays than the standard percentage skill checks. Let us know how it goes and feel free to ask any questions as you go.