Mantra: rendition issues with IE

Hi,

While my website is working fine with FF, everything seems out of place in IE. If I switch to my previous theme (twentyeleven) everything goes back to normality so it seems an issue with Mantra.
Could you please suggest what I can do? I would prefer using Mantra but how the website looks in IE is really bad.
Thanks
My website: https://www.ottimizzazione-pc.it

https://www.ads-software.com/extend/themes/mantra/