Creating custom themes for your store is generally pretty easy! This does require a little bit of CSS knowledge.
Create a new theme file, name it whatever you want just make sure it ends in .css
.
Write whatever CSS changes you wish into the file that you would like to be replicated on your store system.
Save the file.
Upload the new theme file (make sure the file name is unique) to the Themes section of the admin panel of your Store.
Select the theme from the themes dropdown and select 'Apply'.
Refresh the page, on-occasion you will need to press CTRL + F5 a few times for the CSS Cache to refresh. In other cases you can use a CSS Refresher chrome extension. This may not always be necessary as within 48 hours the website should show all changes effectively across all browsers and platforms.