I followed these steps and it works for me:

1. Copy Dropbox link of CSS file
2. Changed dl=0 to raw=1
3. Followed the way of @Mordecai to change www. to dl.

It will look like this:

<link rel="stylesheet"
href="https://dl.dropbox.com/scl/fi/xxx/styles.css?rlkey=xxx&raw=1">