Why won't my css stylesheet work in Sinatra?

I'm relatively new to this but as far as I can tell from my research, this should be working. I'm trying to link a css stylesheet to an .erb file. Here is the code I'm using:

<link rel="stylesheet" type="text/css" href="../css/stylesheet.css">

I've also tried

href="./css/stylesheet.css"

and

href="css/stylesheet.css"

for reference my erb file and css file both sit in different subdirectorys of the same main directory. Here is the code contained within my stylesheet, just in case this is the problem:

body { 
  border-style: solid;
  border-color: black;
  border-width: 5px;
  text-align: center;
  background-color:powderblue;     
}

h1 {
  font-family: verdana;
}

h2 {
  font-family: sans-serif;
}