How to use existing custom UI theme after upgrade to Atlas 2

Hi,   We are upgrading Mx7 projects to Mx8 and Atlas 2. These projects have their own custom theme. We followed the instructions on Mendix docs: and   However somehow after the upgrade to Atlas 2, the custom theme is no longer used.   On the Mendix troubleshooting pages I read the following (vague) comments:   “3 Streamlined Custom Themes Before Mendix 8, the client provided a large amount of default styling if your app lacked a theme. This made building your own theme difficult, as you needed to override the default styling. As of Mendix 8, all styling has been moved to AtlasUI. Now, building your own theme from scratch requires significantly less work. If you have already built your own theme from scratch in an earlier version of Mendix, you might depend on the default styling (specifically the Bootstrap files and the mxui.css file) not included in Mendix 8 applications by default. For this case, Mendix provides legacy mxui.css and Bootstrap files with defaults in this GitHub repository. Download files from this repository to enable your custom theme.”   Downloading these legacy files from the Github repository is not enough. Should we download the whole folder structure from the Github repository? Where should we put these folders and/or files in the project directory?   Are we dependent on these legacy files, or is there another way to use our custom theme in combination with Atlas 2?   Our knowledge about custom theming is limited. Our existing theme was created by external consultants, most probably based on the old Mendix UI Framework, when Atlas didn't even exist.   Kind regards, Elco Fransen
2 answers

Hi Eelco, running into the exact same issue, have you found some answers to your questions perhaps?


Kind regards,



adding your old stylesheet seems to be the correct path.

  1. Did you make sure all files are imported in the main.scss file?
  2. have you already used a compile tool as calypso? This is still mandatory in Mx8 (mendix 10 and onwards this is automated)
  3. tried a simple test to adjust the background in your app within the same folder structure? then atleast you’ll know if something happens after compiling
  4. plenty of widgets have adjusted html stuctures, classes etc. so it could be some stylesheets have to be upgraded
  5. checked the layouts, are you using your old layouts or the newly upgraded ones? Also on this level their could be changes
  6. check the custom-variables, these have also been adjusted $color-primary → $brand-primary so if in the old variables are used they might need to get updated