You don't need a .yaml file at all. A single CSS file can constitute a plugin.
You can simply create a file:
/plugins/myplugin/css/style.css
...then activate "myplugin" from Plugin Manager (it will go in the "General" category, as there is no .yaml file to define the category). Feel free to choose a name other than "myplugin".
Then your CSS will be included after the main CSS.
Dan