Hi Anjum ,
You can find file on theme folder->wp-content->themes->sleey-pro->sleeky_includes->sleeky_customizer.php
There on sleeky_customizer.php you can find code for sticky menu on line no 1855 to 1888 from where, inline css is added on #sleeky_header using .css js event handler.
Hope this works for you, if not please come on online so we can discuss on detail and we can provide you appropriate solution.
Thanks