How can designers contribute to open source?
One way of contributing is to open an issue on the project repository and tell maintainers about your idea. You can also report bugs related to the project and they are counted as a contribution as well. You can also help define a style guide to help the project have a consistent visual design.
What is open source design?
Open-source design — is a design artifact (library, guide, font, code) that is published under an open source license, and is [usually] available on GitHub in the form of source files that can be modified freely. Examples: Google Fonts, few font foundries, Firefox Browser, Unsplash etc.