You can see a customize icon on the lower right corner of your blog or you can try this link to go to your customization page – https://shivashaktiselftodivineself.wordpress.com/wp-admin/customize.php?theme=pub%2Fhemingway-rewritten
On the left side you can see the CSS Tab and after clicking it, you will have the option to add a custom css, for example if you want the background to be green (#008000) using CSS colors, you can put in:
To learn more about custom CSS, see – https://en.support.wordpress.com/custom-css/
To learn more about hex colors, see – https://en.support.wordpress.com/custom-design/custom-colors/#color-picker
To learn more about customizing your blog – https://en.support.wordpress.com/customize/
Let me know how did it go ok?