Describe the bug

Having one of the dark themes enabled in the GitHub settings, the "pipx" part of the logo is barely in the repository README. I've got some screenshots to demonstrate this:

Dark dimmed theme:

Dark default theme:

How to reproduce

1. Enable Dark Mode on GitHub (https://github.com/settings/appearance)
2. Go to https://github.com/pypa/pipx
3. Scroll to the beginning of README just below the file list
4. See the left part of the pipx logo being hardly distinguishable from the background

Expected behavior

The "pipx" text part of the logo should have a good contrast ratio against the background. It should work with all the themes GitHub allows the end-users to have. As a bonus, it'd be nice to check it against the popular https://github.com/StylishThemes/GitHub-Dark user style too.

P.S. If somebody has a vector version of the logo, it'd be good to make use of that instead of a bitmap PNG — SVGs are well-supported in web environments nowadays.