In the first post, there are the instruction for creating and installing custom CSS.
What you did won't work because:
The name
docker-custom.css
doesn't follow the naming pattern for the plugin.
You put the CSS file in the scripts folder, you should put the CSS files inside the styles and JS files inside the scripts folder.
(here and here for reference)
.folder-preview {
height: auto;
}
.folder-preview-wrapper {
margin: 10px;
}
Those are the custom rules you need to apply, now save those into a file. Go to the settings tab on unraid, open the Folder view page, now you should see a button called Open File Manager, click that, and it should open the configuration folder for the plugin. Now open the styles folder (if it don't exist, create it) and upload the file (remember to name the file with the right name).
And you are done.