FOR VERSION 3.0.9 OR EARLIER ONLY
Add it under Semplice > Advanced Styling > Custom CSS & JS
Under this section, you will find the following:
Add any CSS here that you would like to affect the entire site, no matter the screen size.
You will find Desktop, Tablet Landscape, Tablet Portrait, Mobile Landscape, Mobile Portrait. You can add CSS to these to specifically target that screen size.
For CSS, when you navigate to Pages > (your page name) > Branding, you will find a box near the bottom titled 'Custom CSS'. If you enter CSS there, it will only affect that page.