Why my website looks different on another computers?

Try clearing cache in your computer

thank you for your help!i cleared the caches but it didn’t help.maybe there is something wrong in style.css?

The theme is designed to look different on different sized devices. Yes, it looks like some of your child theme CSS is likely causing problems. You should ask on the Mantra forum about this –

https://www.ads-software.com/support/theme/mantra