I wasn't able to find much on contributing to an existing plugin. Is there anything to lookout for e.g. the plugin's repo has a CONTRIBUTE.md docs. Or should I just make a pull request?
The plugin: OctoPrint-CustomBackground
Proposed feature: Add selection to "Image Fill Method" - "Custom Value", then additional input type="text" tag to input value i.e. 50%, which sets the background-size CSS property.
Additionally - while I'm here, if I wanted to make a variation of an existing plugin; giving it several new features, but using a current plugin to start off with - is that a thing i.e. yes, but contact existing owner?
I would personally rather see contributions to the original plugin, unless it seems abandoned. In which case there is a procedure for reporting that on the plugin repo, and adopting it if you are willing to take responsibility in maintaining it.
Every plugin owner will probably have a slightly different answer but from my perspective, I would welcome a well written PR for my addon. I also would respond favorably to a well written issue.
If the PR is a substantial variation of an existing plugin, then I would contact the owner (via an issue?) and discuss your changes. If the owner agrees, then open a PR. If the owner would prefer the leave the existing plugin alone, then create a new plugin with a new name and submit that. I'd include a credit to the original owner in the new plugin.