Advancement Background

yo! Ive never made actuallya advancements that werent used to trigger something. So i have two questions that i could use some help with. I'm pretty sure there both easy fixes but I've been stuck for a little bit now T-T. 1. I would like to fill in the background with an actual block texture instead of a missing texture. 2. Is there anyway to have an advancement tab that exists automatically when you start a new world. Any help is appreciated ^~^