Ajuda, não consigo sair dessa parte

Ok, to help though we need to see your code.
There is a sample in the instructions that show you how to write the selector and add the property / value pair that’s needed.
And a previous lesson showed how you add the class attribute. There are also examples of the class attribute in the current code.

To show your current code follow these instructions…
When you enter a code block into a forum post, please precede it with a separate line of three backticks and follow it with a separate line of three backticks to make it easier to read.

You can also use the “preformatted text” tool in the editor (</>) to add backticks around text.

See this post to find the backtick on your keyboard.
Note: Backticks (`) are not single quotes (’).