How do I create custom defaults for new documents?

When I create a new diagram I always have to reset a bunch of colors and line styles. How can I change the defaults to something more appropriate to my work?

I would start with:

File > New Resource > New Template

and create a template or two of your own, with the settings you need.

Ok. But how do I set the default border color for new lines and shapes? or default text size and color? I would like to start a new blank document and use the drawing tools or outline to create new diagrams, rather than copy/paste elements from the template. Like, so when I create a new connecting line using the drawing tool it is by default an orthogonal line with an arrowhead instead of curved.

Each of the styles used in the Template canvas (for Shapes, Connectors, Text etc) will be available to be selected as Current in the dropdown of the Tool icon.