title: Changing border color in mat-form-field
body: <p>I am using angular material <code>mat-form-field</code>. I have a dark background, and therefore am trying to change the border of the form-field to white. But I am not able to achieve it using css. No, matter what changes I do in the css, they are not reflecting back into the <code>mat-form-field</code>. Here is the link to my code:</p>
  <p><a href="https://stackblitz.com/edit/angular-nnzlr5" rel="noreferrer">StackBlitz link to my work</a></p>
  <p>Any help would be highly appreciated.
  Thanks.</p>