Convert less to stylus files. Note that less scoping will not work in stylus as stylus does not have scoping.

less2styl.js

var fs = require('fs');

function less2stylus(source, mixinNames) {
    return source

        // @var => $var
        .replace(/@([a-zA-Z0-9-_]+)[\s\t]*(:?)[\s\t]*/g, function(str, key, sep) {

            // do not convert import or media keywords
            if(['import', 'media'].indexOf(key) > -1) { return str; } 

            // replace @ with $
            if(sep) {
                return '$' + key + '=';
            } else {
                return '$' + key;
            }
        })

        // @import "foo.less" => @import "foo.styl"
        .replace(/@import[\s]*["']?([^"']+)\.less["']?/g, function(str, fileName) {
            return '@import "' + fileName + '.styl"';
        })

        // .mixin() => mixin()
        .replace(/\.([a-zA-Z0-9_-]+)[\s\t]*(\(?)/g, function(str, mixinName, paren) {
            if(mixinNames.indexOf(mixinName) > -1) {
                if(paren) {
                    return mixinName + paren;
                } else {
                    return mixinName + '()';
                }
            }
            return str;
        })

        // ; => 
        // : =>
        // { =>
        // } =>
        .replace(/;[^\n\/]*([\n]?\/?\/?)/g, function(str, trail) {
            return trail;
        })
        .replace(/:/g, '')
        .replace(/\{/g, '')
        .replace(/\}/g, '')
        .replace(/[\s\t]*\n/g, '\n');
}

function catalogLessMixins(source, mixinNames) {
    var mixinName = true;
    var regex = /\.([a-zA-Z0-9_-]+)\(/g;
    while(mixinName) {
        var mixinName = regex.exec(source);
        if(mixinName && mixinName[1] && mixinNames.indexOf(mixinName[1]) == -1) {
            console.log('Mixin "' + mixinName[1] + '"');
            mixinNames.push(mixinName[1]);
        }
    }
}

function catalog(src, mixinNames) {
    var srcStat = fs.statSync(src);

    // directory
    if(srcStat.isDirectory()) {

        // iterate
        var srcDir = fs.readdirSync(src);
        for(var iD = 0; iD < srcDir.length; iD += 1) {
            catalog(src + '/' + srcDir[iD], mixinNames);
        }
    } 

    // file
    else if (srcStat.isFile() && src.match(/\.less$/)) {
        // convert to stylus
        var source = fs.readFileSync(src, "utf-8");
        catalogLessMixins(source, mixinNames);
    }
}

function convert(src, dest, mixinNames) {
    mixinNames = mixinNames || [];
    var srcStat = fs.statSync(src);

    // directory
    if(srcStat.isDirectory()) {

        // check for the dest directory
        if(!fs.existsSync(dest)) {
            fs.mkdirSync(dest);
        }

        // iterate
        var srcDir = fs.readdirSync(src);
        for(var iD = 0; iD < srcDir.length; iD += 1) {
            catalog(src + '/' + srcDir[iD], mixinNames);
        }
        for(var iD = 0; iD < srcDir.length; iD += 1) {
            convert(src + '/' + srcDir[iD], dest + '/' + srcDir[iD].replace(/\.less$/, ".styl"), mixinNames);
        }
    } 

    // file
    else if (srcStat.isFile() && src.match(/\.less$/)) {

        console.log(src + ' => ' + dest);

        // convert to stylus
        var source = fs.readFileSync(src, "utf-8");
        var output = less2stylus(source, mixinNames);
        fs.writeFileSync(dest, output);
    }
}

convert('./less', './styl', [/* any mixins that are never used with () */]);