(in your individual Projects)

General

• Extend settings and tools only with definitions that has to be global
• Split, Split Split. Everything you can into useful and reusable (style)-components
• Avoid writing super long stylesheets. There is no length restriction but take care of your own code and of some that needs to work with it.
• Avoid making super long nested rules. Split for readability!
• Write useful comments to e.g. structure your document
• Avoid using hard-coded colors that are not a part of your global color-settings.
• Keep CSS Specificity low!

Formatting

• Write shorthand properties!
• Skip units that are not necessary. margin: 0;
• Do not use Id's for Styling purpose.
• Avoid leading Zeros. font-size: .8em;
• You don't need to prefix CSS properties. There is an auto-prefixer by default that sets prefixes according to your browserlist in the package.json.
• Use THE app-mq(); Mixin
• Use single CSS Quotation Marks background-image: url('myimage.jpg')
• Write only lowercase & kebab case class names .o-max-block-width

Last Words: Think twice and write only CSS that has an impact by cleaning up unnecessary stuff after you got it working!