Cannot change BG colour in bootstrap Navbar

Hi everyone

I am a complete beginner in DW this question might sound stupid but I could not find anything on the internet for this specific problem.

So, I am trying to create a website, with Bootstrap, and I have a couple of questions. 

How do I change the BG colour of my navbar? I have followed a tutorial on YouTube but, despite I do everything the guy does my colour won't change.

Created a new style.css sheet, added a selector for the navbar, and changed BG colour to red, but as you can see the colour is still White. 

thanks for any help you can provide

This is the code:

<nav class="navbar fixed-top navbar-expand-lg navbar-light bg-light"> <a class="navbar-brand" href="#">Logo</a>
          <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent1" aria-controls="navbarSupportedContent1" aria-expanded="false" aria-label="Toggle navigation"> <span class="navbar-toggler-icon"></span> </button>
          <div class="collapse navbar-collapse" id="navbarSupportedContent1">
            <ul class="navbar-nav mr-auto">
              <li class="nav-item active"> <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a> </li>
              <li class="nav-item"> <a class="nav-link" href="#">Link</a> </li>
              <li class="nav-item dropdown"> <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown1" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false"> Dropdown </a>
                <div class="dropdown-menu" aria-labelledby="navbarDropdown1"> <a class="dropdown-item" href="#">Action</a> <a class="dropdown-item" href="#">Another action</a>
                  <div class="dropdown-divider"></div>
                  <a class="dropdown-item" href="#">Something else here</a> </div>
              </li>
              <li class="nav-item"> <a class="nav-link disabled" href="#">Disabled</a> </li>
            </ul>
            <form class="form-inline my-2 my-lg-0">
              <input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
              <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
            </form>
          </div>
        </nav>