Kde titlebar icon customisation, to add resize icon

I am using kde 5 with oxygen theme.

I like this theme. However, like most themes, lacks a resize button in the window title bar.

I have looked into customizing the theme, but have had no luck:
I can not find where the title-bar icons are stored.
I can not figure out any documentation. All of which seems to start out simple then suddenly drops deep in, it assumes that I know what it is talking about, it does not even say what language the code is in, or where it goes.

Can anyone tell me where to start?
Do I just need to add an icon? If so how?
Do I need to do more to enable the functionality? (note the functionality is found in the horribly ugly “BII preview” theme, but non of the nicer themes. )

I am using Gnu/Linux, Debian 10 (Buster), and plasma 5