I have an svg image, generated by github stats.

At the end of its svg url, you can add an anchor to set the theme of the svg, since the svg includes css.

example: https://raw.githubusercontent.com/ThomasAunvik/github-stats/master/generated/overview.svg#gh-dark-mode-only

And this is the url next/image generated:
/_next/image?url=https%3A%2F%2Fraw.githubusercontent.com%2FThomasAunvik%2Fgithub-stats%2Fmaster%2Fgenerated%2Foverview.svg%23gh-dark-mode-only&w=640&q=75

This makes it so the SVG doesn't have an anchor point. Add the anchor at the end of the next url image, and it would work, example

<!-- This shows light mode -->
<Image
  src="https://raw.githubusercontent.com/ThomasAunvik/github-stats/master/generated/languages.svg#gh-dark-mode-only"
  width={300}
  height={100}
  alt="github-languages"
/>

<!-- This shows dark mode -->
<img src="/_next/image?url=https%3A%2F%2Fraw.githubusercontent.com%2FThomasAunvik%2Fgithub-stats%2Fmaster%2Fgenerated%2Flanguages.svg&w=640&q=75#gh-dark-mode-only" />