A WordPress theme includes a style sheet containing CSS (Cascading Style Sheets) code that determines the appearance of the elements of the website – from the size and colour of the text, to the way the images line up on the page.
For example, if I want to add a blue dashed border around my images, as shown below, then I could do it with this CSS:
“border: dashed 2px #4c5ea0;”
This code gives my next image a different look:
“-webkit-border-radius: 10px; border-radius: 10px;
-webkit-box-shadow: 4px 4px 15px 5px #808080; box-shadow: 4px 4px 15px 5px #808080;”
As you can see from the examples above, CSS code can be used to customise a website.
It is possible to directly edit a theme’s style sheet but this isn’t recommended (unless you are using a child theme) since the changes will be lost if an updated version of the theme is installed. This could happen if the theme’s developer brings out a new version to fix some bugs or add improvements.
If you are not using a child theme, you can add the CSS code in the Additional CSS area of the Customiser. Alternatively, you could use a plugin such as Simple CSS.
For more information, see https://www.wpbeginner.com/plugins/how-to-easily-add-custom-css-to-your-wordpress-site/.