Slider is working fine, it is just that background image is set to cover entire container which will cause clipping on smaller screens. You can improve it by adding images where important part of the image is in center of it.
Regarding mobile menu button you should change it to some color other then white https://www.screencast.com/t/PBMFeM3NgT . Off course this can be done with css code below also:
color: #000 !important;
You can apply the code from Appearance > Customize > Additional CSS field and result should look like this https://www.screencast.com/t/2yM8y0hLYos4