I’ve played with the available icons for a bit. Many are just too small or indistinct in this situation, but hopefully I’ve found one that works.
Try the attached replacement (extract from the zip file first) for /modules_v3/themes/simpl_grey/style.css and let me know what you think.
Your question about zoom-in / zoom out is an interesting one. Those two are in use elsewhere (on chart boxes). But the magnifying glass is also used for “search” which seems a bit confusing. But I’ve yet to find an alternative for zoom (or search) that is better.