Vanilla Art Fixes

Makes tweaks and fixes to the themes and art of vanilla cards.

All cards now use the same background and foreground prefabs within a theme. The following cards have had their theme changed:

Big Bullet -> Destructive Red Buckshot -> Firepower Yellow Drill Ammo -> Tech White Echo -> Defensive Blue Implode -> Magic Pink Leech -> Nature Brown Saw -> Tech White Shockwave -> Destructive Red Taste of Blood -> Evil Purple Teleport -> Magic Pink