If changing the icons on the LG device is still very easy, but there is no function to reset icons back to default, so the theme (design) must be set back/again to default to use LG's standard icons.

Note: Unfortunately the background will be set back to standard and may need to be personalized again.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  Display
Step 3:  Tap on  Wallpaper & theme
Step 4:  Tap on  Theme
Step 5:  Tap on  Default Theme
Step 6:  Tap on  Apply
  1. Open the Settings
  2. Tap on Display
  3. Tap on Wallpaper & theme
  4. Tap on Theme
  5. Tap on Default Theme
  6. Tap on Apply

LG Instructions