Resolve stylelint complaints

Problem/Motivation

Now that 📌 Enable GitLab CI Fixed and #3290112: Port to Drupal 10 and 11 → are done, our GitLab CI pipelines are warning about the stylelint job, which is currently failing. E.g.

https://git.drupalcode.org/project/toolbar_responsive_search/-/jobs/6907857

css/toolbar_responsive_search.css
   6:43  ✖  Expected double colon pseudo-element notation  selector-pseudo-element-colon-notation
   9:50  ✖  Expected double colon pseudo-element notation  selector-pseudo-element-colon-notation
  10:53  ✖  Expected double colon pseudo-element notation  selector-pseudo-element-colon-notation
  42:30  ✖  Insert "·"                                     prettier/prettier
✖ 4 problems (4 errors, 0 warnings)

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes