Customising Twenty-Fifteen

Does anyone know how I can customise TwentyFifteen to (1) reduce the font-size and (2) change the colour set? It claims to have 5 different colour sets but I can't make them work.

If you open the Themes menu in Admin, and click the theme, you'll see an option to change the color scheme and customize other colors. Font size could be reduced using the Custom CSS plugin.

Thanks Flavio.

Colours: I can successfully set custom values via the Admin/Themes but there's a drop-down called 'Color Scheme' with values like Dark, Yellow, Pink, etc, which seems to have no effect. Is there a switch I have to set somewhere?

Custom CSS: installed and enabled. I set 'Inline CSS' to .entry-content {font-size: 80%; } but I get a Twig error,
"Undefined index: path"
What am I doing wrong? Quite important for the client to be able to reduce font-size, so I need to make this work.