Hi Nick,

You can use by these ways.

Using inline CSS.

// Demo Component.cmp

<aura:component>
  <div style="background-color:black ; margin:12px">
    <p style="color:white">white Text with black Background [Inline CSS]</p>
    <p style="color:yellow">Yellow Text with black Background [Inline CSS]</p>  
  </div>
</aura:component>


//By create STYLE tab in component bundle.

//Demo Component.cmp

<aura:component >
  <!--here div is top level elements and <h1> and <p> is nested level elements-->  
   
   <div class="divCss">
      <h1>Hello Developers's</h1>
      <p>This is sample HTML Paragraph in a DIV elelment.</p>
   </div>
   
  <!--here <p> tag is top level elements-->
    <p>This is a sample HTML paragraph.<span>with a span element</span></p>  
</aura:component>


// Put this code to Demo Component.css tag.

/* For select ALL TOP level <p> element */
 
p.THIS {
    color: blue;
    font-weight: bold;
}
 
/* For select ALL TOP level <Div> element with class Name "divCss" */
 
.divCss.THIS {
    padding: 20px;
    background-color: yellow;
}
 
/* For select ALL Nested level H1 elements */
 
.THIS H1 {
    color: green;
    font-size: 22px;
}
 
/* For select ALL Nested level <p> element */
 
.THIS p {
    color: red;
    font-weight: lighter;
}

Please select as best answer if it helps you.

Thank You,
Ajay Dubedi