I’d give the HTML element a class, then put your CSS in a central location like:
– The CSS editor included in the Customizer in WordPress 4.7
– Install the Jetpack plugin and activate the Custom CSS module
– Install a custom CSS plugin like https://ww.wp.xz.cn/plugins/simple-css/
– Create a child theme
Do I have to create a child theme?, wouldn’t the first three steps suffice?
You only need to do one of those options. If you only have a relatively small amount of custom CSS, the WP custom CSS editor is a good option, and the CSS stays with the theme. If you have a moderate amount of code then a plugin is a good option. For a large amount of custom CSS I’d add a child theme.
You’re welcome; glad to help.