I don't have experience with using Awesome Widgets, or other similar tools. I'm just aware that they exist, having seen them pop up on the kde store some time ago.

As for the black text on black background problem when using breeze dark, you may be able to resolve that by digging into the widget's settings found in ~/.local/share/plasma/plasmoids/<widget's.folder>. It sounds like they just declared the text color to be black, assuming a light color scheme.

For tutorials on writing or editing plasma widgets, KDE Developer David Edmundson wrote a series of articles on that:
http://blog.davidedmundson.co.uk/blog/p ... utorial-1/
http://blog.davidedmundson.co.uk/blog/p ... ting-data/
http://blog.davidedmundson.co.uk/blog/p ... ending-in/