Administration - Installation - Core: To answer my own question

If you need a new property add it to ../myskin/myskin-variables.less

     @my-property: some-val; 

This allows the System Skinner to pick up the property for configuration in the GUI. Create a new LESS file ../myskin/my-custom-properties.less and fill it with plain CSS and import it in ../myskin/myskin.less or just append the CSS to ../myskin/myskin.less. Both works. 

    .my-property {
        property: @my-property;
    }

Rebuild your CSS. Done.