If you prefer to use a classic font like Arial instead of the system font or if you experience some problems with the system font in Matomo (Piwik) 3 install this theme.
How do I activate this theme?
Once you have installed and activated this plugin, go to "Administration => Plugins => Themes" and there you can activate it.
Will all users of my Matomo (Piwik) installation use this theme once activated?
Yes, in Matomo an activated theme always applies to all users.