This is a small (5.1KB) custom theme for Datasette, inspired by my previous work on the Alexandria library (a curated collection of articles from Hacker News). It provides a minimalistic and grayscale-based design. I consider it as a trade-off between the functionality of the default Datasette theme and the aesthetics of this custom theme. It might not be your cup of tea, but I hope you'll like it!
The theme is available at https://cdn.jsdelivr.net/gh/julien040/charcoal-datasette-theme@1.0.0/theme.min.css.
Datasette is well made and allows you to add custom CSS to your instance. You can do so by adding the URL to the extra_css_urls array in your metadata.json file.
{
"extra_css_urls": [
"https://cdn.jsdelivr.net/gh/julien040/charcoal-datasette-theme@1.0.0/theme.min.css"
]
}
The theme also works on mobile devices, here is a screenshot of the table view on a mobile device:

This theme is released under the MIT license. See the LICENSE file for more information.
I think the work is mostly done, but if you have any suggestions or improvements, feel free to open an issue or a pull request! I haven't tested the theme on plugins, so if you encounter any issues, please let me know.