Drop down menu is hidden behind a slider.

Hello on my company’s website https://www.options-inc.org (when using the mobile version of our site) the drop down menu becomes hidden behind the slider on the main page. I have attempted to adjust the z-values in our theme, but I am not entirely sure where to begin. I am by no means a programmer and only dabble with coding. I went into our themes css file went to where I thought the dropdown menu was located and adjusted the z-value from 9999 to 99999 and nothing seemed to change. Anyone that could shed some light on this issue would be much appreciated. Thank you.