Here you defined an STATIC_ROOT, so you after placing your static files and on each change of your css or js files you need to run
python manage.py collectstatic
This command will gather all static files into the defined dictionary, but this is done after the completion of development. So instead of that you can use STATICFILES_DIRS during development,
STATICFILES_DIRS = [ os.path.join(BASE_DIR, "static") ]
which will automatically find the static files during your development period.
Also make sure you have staticfiles in installed apps.
INSTALLED_APPS = [ 'django.contrib.admin', 'django.contrib.auth', 'django.contrib.contenttypes', 'django.contrib.sessions', 'django.contrib.messages', 'django.contrib.staticfiles', # it's is used for static files ]
In your template you can load static files like
{% load staticfiles %} # register static tag <link ref="stylesheet" href="{% static 'style.css' %}">