You can change icons in header in theme on another RH icon.
You can find code of icon on site. Then, copy code of icon and replace content of icon in code.

To change cart icon
.rh_woocartmenu-icon:before{content: "\f640";}
To change wishlist icon
#main_header .rhi-hearttip:before {content: "\f640";}
To change Login icon
#main_header .rhi-user:before {content: "\f640";}
To change comparison icon
.re-compare-icon-toggle:before{content: "\f640";} .re-compare-icon-toggle .rhi-shuffle:before{content: "\f640";}
To change Plus icon for button
#main_header .rhi-plus:before {content: "\f640";}
To change Sign In icon on login button
#main_header .rhi-sign-in:before {content: "\f640";}
For example, you want to change cart icon. So, copy
.rh_woocartmenu-icon:before{content: "\f640";}
and replace f640 to code of new icon. You will get something like
.rh_woocartmenu-icon:before{content: "\f07a";}
Place it in Theme option – general – custom css