Add open-props topic

[SrGeneroso]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• I am not the sole author or employee of a company who created either the topic I am modifying/adding or the collection entry I am modifying/adding.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288, size <= 75 kB)
• Content (and my changes are in index.md)

Please replace this line with an explanation of why you think these changes should be made.

Curating a new topic or collection

• I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/[NAME] or https://github.com/collections/[NAME])
• My folder contains a *.png image (if applicable) and index.md
• All required fields in my index.md conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

Open Props is a very thoughtful and comprehensive design tool that manages to be easy to use, beautiful and fully compatible with any project that uses CSS. It was made by Adam Argyle, who is one of the best advocates for CSS (He is currently working on google for the CSS group for chrome). Please consider this entry as it is the best way I can find to show some love to the project and the awesome community around it.

Something that does not neatly fit into the binary options above

• My suggested edits are not about an existing topic or collection, or at least not a single one
• My suggested edits are not about curating a new topic or collection, or at least not a single one
• My suggested edits conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

Please replace this line with an explanation of your proposed changes.

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Please note: we will close your PR without comment if you do not check the boxes above and provide ALL requested information.

[jmeridth]

Couple requests. Thank you.