Here's a link to the build preview of that section:

https://python-docs-theme-previews--166.org.readthedocs.build/en/166/reference/datamodel.html#object.reversed

There is a small gap between the underscore and underline (Android/Chrome):

macOS / Chrome:

0.2em

0.21em

0.22em

0.23em

0.24em

0.25em

This is on Linux/Firefox:
0.2em:

0.25em:

0.3em:

I like 0.25 -- with 0.2 I have to do a double take because I'm not sure if there are two lines or I'm seeing double; with 0.3 the difference is clear, but it does look a bit too distant (especially when there are no underscores); with 0.25 the underline is closer to the text but I can still see the two lines distinctly. (Disclaimer, I'm running on 4h of sleep and my eyes are a bit tired, I can take a look again tomorrow -- maybe the 0.2 will look better).

0.25em looks okay to me, PR updated.