You can change all the theme colours in the Appearance » Customise section.
In this section you can change all the main elements colours and you can preview the changes while selecting your colours:
The accent colour is the primary Site elements colour, which is applied as background or text colour to different elements, such as buttons colour, elements hover colour (e.g. arrows), pagination elements colour, etc. You can change the default Accent Colour in the General Colour Settings » Accent Colour section of the Theme Customiser panel:
Other style options
The rest of the Theme Customiser sections contain some general style settings, such as colours for the main header, content and footer areas.