Remove a workaround for a CSS issue in older Firefox versions, which was
recently fixed (at some point between Firefox 102 and 108). Firefox 108+
now displays the bullet points at the right vertical position even without
the hack in [1].

[1] https://gluon.readthedocs.io/en/latest/releases/v2018.1.html#site-conf

Add a custom CSS class to support strike-through text without depending on
a specific theme.

Fix a number of issues related to the vertical alignment and margins of
list elements.