Similar to Xiaomi, Oppo has a built-in theme app that allows you to customize the lock screen, wallpaper and icons. By default, you have the ColorOS and Jelly Bean themes to choose from.
Oppo does not keep count on the total number of available themes. There is a constant fluctuation in the numbers in lieu of fan-made themes that are made available or taken down. The ones you see in the Theme app are curated by Opoo.
You will be spoilt for choices looking through the database of themes in ColorOS.