Hi qouiou , as you can see the
.ibs_ stylesheet has not been closed. It looks like the IBS abbreviation stand for your own website name. So I guess that you or your developer has added the code but forgot to close it.
It has nothing to do with your
style.css but to a codeblock that hasn't been closed. This eventually could happen due to cache, installed/updated plugins or via a custom header/footer plugin where you forgot to close the
<style> tag yourself.
Do you have a custom header/footer plugin installed?