The first two options should not be used(unless you really know what you are doing) for two important reasons.
- It is possible to make the entire Foundry/Sandbox experience unusable.
- Any changes to them will be overwritten when updating to newer versions
The official Sandbox way to use is the Custom Sandbox World Style Sheet.
However the easiest way to work with CSS in Sandbox (and Foundry) is to use the module [Custom CSS Rules](https://foundryvtt.com/packages/custom-css)
### Custom Sandbox World Style Sheet
Create a text file called custom.css(the actual name of this will not matter) in your world folder with a text editor. A simple editor like NotePad will work but to make it easier there are many editors out there that have support for CSS syntax highlighting like Sublime, NotePad++ or Synwrite.
Add CSS content to the file and save it.
In Foundry, open up **Configure Settings** and in the **System Setting** tab, in the **CSS Style file** field, add the path to your CSS-file.
In the example below, the file custom.css is residing in the world *Oneshots*, in a subfolder named *style*
When working with this method, every time you make changes to the CSS-file, you need to reload the Foundry client(pressing F5). This can be a bit tedious, which brings us to the next method.
### Module Custom CSS Rules
This module makes it possible to modify and test your CSS without having to press F5 constantly(and thus closing whatever windows that are open every time.).
Each time you press Save in the Custom CSS Rules dialog, your content will be automatically updated.
When you are satisfied with your styling, you can leave it like it is or copy the CSS to the Custom Sandbox World Style Sheet CSS file(as described above)
