To alter the stores theme colour, from the store dashboard go to:
> Store Settings > Store Settings > Branding

From within this page you can either manually alter the stores theme colour by selecting your preferred colours below (once you're happy with the alterations, click 'Save Changes'):

Or you can use a preset of colours. Open the drop down menu underneath the heading 'Use Preset', select which theme colour you'd like and click 'Load Preset' when you're happy with the selection:

(*Don't forget to click 'Save Changes' after making alterations*)