Hello- Is the changes of website design impact on its ranking?
No website design has nothing to do with rankings. Unless it has completely ridiculous navigation system.
I think Little bit affect.
If it is a new website then you can change theme as well. But once it is getting traffic it is better to stick to the same theme as because it distracts visitors interaction and helps you to loss some traffic.
I would say yes! If you will make the web design changes which effect the web page load speed then it's impact is on the rank of your website.
For me, as long as you always check the page speed and validate your website after the changes it doesn't matter, but there's a little bit of effect on getting traffic I think.