Removing borders in "Manifesto" theme

Hi

i just installed “Manifesto” theme on my website but the problem is there’s a thick border on every image which doesn’t look good.

i did a little research and i just found some topics which they were saying to find

div.cover img {
border: 5px solid #333333;
}

and change 5px solid #333333 to none.

but the problem is there’s no “div.cover img” code in my CSS. what should i do? please help me.

by the way the version of my theme is “1.4.5”