Latte Dock v0.10 now allows you to adjust the background roundness. You need the latest version of Latte git, Kde Framework> = 5.69 and several kirigami2 libraries installed for it to work properly. You can also adjust the background shadow size if you want.
Latte "follows" the Plasma Theme almost everywhere and any extended feature should not violate theme design decisions. If a user wants a different background color, it will probably also mean different dialogs, dialog shadows, toolbox colors, etc. Therefore, the background color cannot be adjusted.