I’d suggest you start by having a look at the custom colour schemes that come with Writr to use as a base for your own. For example, open the file css/green.css and take a look.
Copy out whatever styles you want to override in your new colour scheme – it could be all of them – and then tweak the colours as you like.
Don’t edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.
Add your new CSS either using a custom CSS plugin – like the Custom CSS module built into the Jetpack plugin – or in a child theme.