MailWizz EMA will allow you to create a gallery of templates that you can use in your campaigns.
Each client has his own gallery of templates, these are not shared.

You can create templates from scratch using the wysiwyg editor, or you can just upload a zip archive containing your template.
If you decide to upload a zip archive, when you try to do this, you will see that there is an example archive that will help you format your archive so that you don't get any error while uploading or while the template is parsed.

You also can decide if the CSS of the template will be inlined or not, this really depends on the way you provide your template.
Most of the times, you will provide your template containing inline style already.

Please note that you can control the maximum size of uploaded template, i would recommend not allowing templates with a size > 1 mb.