I think we're missing some info here, but you should use a relative path so it is looking in a directory relative to your project:
<link rel="stylesheet" href="{% static 'css/main.css' %}">
This will look for the stylesheet called 'main.css' in a folder using variables from your settings.py file.
Your STATIC_ROOT is where your static files will end up after running python manage.py collectstatic, STATICFILES_DIRS is the folder that collectstatic is going to get your static files from. STATIC_URL is the URL prefix that your static files will come from.
So if you have in settings.py:
STATIC_URL = '/static/'
STATIC_ROOT = os.path.join(BASE_DIR, 'staticfiles')
STATICFILES_DIRS = (
os.path.join(BASE_DIR, 'static'),
)
Your directory should look like this: /your_project_folder/static/css/main.css
And the template tag above will find your main.css file.
<title>? If so, please read up how to apply CSS to HTML. It should have anidorclassapplied as such<title class="title-red">Portfolio</title>