Add /assets/css/default_skin.css in main root template

gulpfile.js

 const _tpldir = 'tracim/tracim/templates/'
 
 const listCssFiles = [
-  _srcdir + 'css/default_skin.css',
+  // _srcdir + 'css/default_skin.css',
   // _srcdir + 'css/bootstrap.css', // Côme - 2018/01/11 - removed to allow easier configuration of theme builder
   _srcdir + 'font-awesome-4.2.0/css/font-awesome.css',
   _srcdir + 'select2-4.0.3/css/select2.min.css',

tracim/tracim/templates/master_anonymous.mak

         <meta name="author" content="">
         <link rel="icon" href="/favicon.ico">
 
+        <link href="${tg.url('/assets/css/default_skin.css')}" rel="stylesheet">
         <link href="${tg.url('/assets/css/bootstrap.css')}" rel="stylesheet">
         <link href="${tg.url('/assets/dist/all.css')}" rel="stylesheet">
 

tracim/tracim/templates/master_authenticated.mak

         <meta name="author" content="">
         <link rel="icon" href="/favicon.ico">
 
+        <link href="${tg.url('/assets/css/default_skin.css')}" rel="stylesheet">
         <link href="${tg.url('/assets/css/bootstrap.css')}" rel="stylesheet">
         <link href="${tg.url('/assets/dist/all.css')}" rel="stylesheet">