thanks, but I am not looking for a workaround. I have one. The code I provided above is incomplete. It was just asuggestion of how maybe one should be able to achieve this"Button Override", including the values in the properties column.
I have a CSS code that works to override the button style. I have another problem.
This custom style is not represented in the properties column in the editor.
E. g. I force the button to be red and square as per corporate design, the properties tab however displays that the button is blue with rounded corners.

What I need is to enter red and square for example in an editor settings file and every time the user adds a button it will have the look of their corporate design and those values represented in the properties column. Which is exactly what the OP is asking for, I think.