Specifically, what I want is a way to edit the color and size of the diamond text marker.

And, yes, the marker size can now be changed. But I want to be able to set a default color and size for all drawings.

Thanks for any suggestions.

https://inkscape.org/forums/beyond/how-to-report-bugs-or-request-new-features/

https://inkscape.org/forums/beyond/signing-up-for-a-gitlab-account-to-report-bugs/

Sounds like a gui related request. Maybe also try asking around in https://chat.inkscape.org/channel/team_ux.

I need to learn to read
 

1.5 is supposed to be able to change handle colors via a built in handle editor. 

Right now - you can change via the handles.css file but it needs more shapes and expanded classes. I have changed both the individual size, color and transparency with a text editor. 
 

I don't think the OP is interested in path markers... My impression is they would like to change the handle for the flow-text box or the text anchor.

https://gitlab.com/inkscape/inbox/-/issues/11200

Being worked on for v1.5