To select wallpaper for LG G6:
Open Settings > Display Tab > Home Screen.
Choose which folder to select a wallpaper.
Select an image.
Resize the image, then tap OK to set.
Note: You will be prompted to set wallpaper to lockscreen, this is optional.
To select theme:
Open Settings > Display Tab > Theme.
Select a theme from the list to preview.
Tap Apply to set.
Note: You can download new themes from the Theme settings but will be prompted to download LG SmartWorld.