enable custom CSS if and only if page does not define any styles in <head>
Enable your custom CSS if and only if page does not define any styles in <head>.
This userscript refers to sp.css from https://github.com/susam/spcss/ project, but you can easily change the link to another style sheet.
Example page without any styles in <head>: https://www.gnu.org/software/termutils/.
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
Screenshots show the same page "before" and "after"
