Even though GitHub Readme files (typically ./readme.md) are Markdown, and although Markdown supports HTML, you can’t put <style> or <script> tags init. (Well, you can, they just get stripped.) So you can’t apply custom styles there. Or can you?
You can use SVG as an <img src="./file.svg" alt="" /> (anywhere).
When used that way, even stuff like animations within them play (wow).
SVG has stuff like <text> for textual content, but also <foreignObject> for regular ol’ HTML content.
SVG support <style> tags.
Your readme.md file does support <img> with SVG sources.
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