As may you aware some applications comes in a dark theme version that break with the light elementary OS theme. If you are looking for how to make the dark apps light to match the rest of the themes this is the place. You can actually set different apps with different themes.
First you need a menu editor for this, type in terminal:
This guide will show you how to install run live wallpapers on elementary OS. NOTE: If you are low on RAM it’s no good idea with live wallpaper and if you’re a gamer make sure you have “gamemode” installed which you can find here.