How do I use tops with arms poses without my avatar's arms showing underneath? I can't remember if there is a special item for this or something.

You can turn off individual limbs under the Base tab in the wardrobe :)

There's also the whiteout pen items, but you won't be able to use those easily with a full background since they cover the limb with white instead of completely removing it (though they would work with skins or items that won't otherwise allow you to remove the limbs entirely):

After opening your wardrobe, click on Base (up near the top). There you'll see options to toggle different limbs on or off.

Sorry for pinging, got beat to it =|

Thank you both!