UCSSPostprocessor

using System;
using System.IO;
using UnityEngine;
using UnityEditor;
using System.Text.RegularExpressions;

class UCSSPostprocessor : AssetPostprocessor
{
    static void OnPostprocessAllAssets(string[] importedAssets, string[] deletedAssets, string[] movedAssets, string[] movedFromAssetPaths)
    {
        foreach (string str in importedAssets)
        {
            if (str.EndsWith(".uss"))
            {
                StreamReader reader = new StreamReader(Application.dataPath + "/../" + str);
                if (reader.ReadLine() == "/* Postprocess */")
                {
                    // Debug.Log("Postprocessing " + str);
                    Regex rx = new Regex(@"(?<=\n)\.(?=\S)",
                        RegexOptions.Compiled);
                    string data = reader.ReadToEnd();
                    foreach(var prefix in Enum.GetNames(typeof(ScreenSizePrefix)))
                    {
                        // Replaces .classname with .prefix--classname
                        string rdata = rx.Replace(data, "." + prefix + "--");
                        string fname = Path.GetFileName(str);
                        string path = str.Remove(str.Length - fname.Length);
                        // Stores variants as prefix_origname.uss
                        StreamWriter writer = new StreamWriter(Application.dataPath + "/../" + path + prefix + "_" + fname);
                        writer.Write(rdata);
                        writer.Close();
                    }
                }
                reader.Close();
            }
        }
    }
}