1. Download Responsive Drop down Menu Coding.

2. Go to Koha Administration › Global system preferences ›OPAC ›

3. Copy the menu source code and paste the following section.
  1.  opacheader  (HTML code)
  2.  OPACUserCSS  (CSS code)
  3.  OPACUserJS  (JS code)
4. Go to OPAC page and refresh (F5)

Finally successes full create Responsive Drop down Menu in your OPAC.

Mobile View