You can do that from the theme’s customizer options by going to your main dashboard admin page, click on the big “Customize this site” button, then on the “Colours” tab in the left column of the customizer, look for the “Main Content Text Colour” setting (default colour is #747474) and choose your colour there. You will be able to see your changes live in the window there before you click Save and Publish.
For settings relating to colours, you can view this tutorial:
https://www.styledthemes.com/preference-theme-setup/57-change-the-colours-of-preference