enable custom CSS if and only if page does not define any styles in <head>
As of
Enable your custom CSS if and only if page does not define any styles in <head>.
This userscript uses sp.css from https://github.com/susam/spcss/ project, but you can easily change it to another style sheet.
Example page without CSS in <head>: https://www.gnu.org/software/termutils/manual/termutils-2.0/html_mono/tput.html.
The script could also be useful for viewing offline documentation, e. g. /usr/share/doc/bash/bash.html
Inspired by https://superuser.com/questions/597260
