title: Customizing an OFfice 365 Sharepoint website with CSS
body: <p>I would like to customize a website hosted on Office 365. For example, I would like to do something as simple as changing the font size on all the pages of the wiki.</p>
  
  <p>I haven't found an option of editing the website CSS in the Site Settings. How can this be done?</p>
  
  <p>(I guess I'll have to edit the current theme or to add a new theme, but I haven't found a guide to do any of these).</p>
  
  <p>Thanks in advance!</p>