Edit layout and setting up blog show!

Please help me change the size border in top body,
when i view “Inspect Element”, i saw :

html {
  margin-top: 32px !important;
}

Could you please let me where is located the file? I would like to change smaller 12px.
here is my website: https://vietnambusinesstour.com/en/

One more question, please let me know how to show blog list as grid view?
https://vietnambusinesstour.com/en/category/business-news/

Please help me.
Thank you very much!