As of eZ Publish 5.0, the admin2 design has been renamed to admin, as documented here:
https://github.com/ezsystems/ezpublish-legacy/blob/master/doc/bc/5.0/changes-5.0.txt

But the same did not happened in eZ Flow LS.
When we use the admin design only, some part of the CSS will try to load a button_bg.png, as we can see in the extension/ezflow/design/admin/stylesheets/ezpage/ezpage.css file:

div.zones .button {
    background-image: url("../../images/ezpage/button_bg.png");
    background-position: center top;
    background-repeat: repeat-x;
    border: 1px solid #d8d8d8;
    color: #ffffff;
    cursor: pointer;
    padding: 0.15em 0.5em 0.2em;
}

div.block-container .button {
    background-image: url("../../images/ezpage/button_bg.png");
    background-position: center top;
    background-repeat: repeat-x;
    border: 1px solid #d8d8d8;
    color: #ffffff;
    cursor: pointer;
    padding: 0.15em 0.5em 0.2em;
}

Steps to reproduce:

1. Install eZ Publish 5.3 with eZ Demo
2. Update it to 5.3.2
3. Edit the ezpublish_legacy/settings/siteaccess/ezdemo_site_admin/site.ini.append.php file
4. Remove the following line: AdditionalSiteDesignList[]=admin2
5. Save the changes
6. Clear the caches
7. Log in to your backend (ezdemo_site_admin)
8. Edit the "Home" Landing Page (ezpage), or create a new one

You will see that the buttons will be missing the background (check the attached screenshot).