Why is there no Customize link?
The customizer works when the public theme is built on the Theme Engine, such as Vitrine, Confetti, Plinth or Overprint. Check Appearance › Themes. The links are also hidden from users who may not configure themes: in Omeka Classic only Super Users may, unless a plugin changes the permissions.
Why do visitors not see my changes?
Changes in the customizer are a draft until you click Publish. Only you see the draft.
I saved on Configure Theme while I had a draft. What happens?
Configure Theme saves its own form. Your draft stays as it was: the settings you changed in the customizer still show in your preview, and publishing applies them over what was saved. Discard the draft if you no longer need it.
The status line says my session has expired. What do I do?
Reload the customizer page (log in again if Omeka asks). Your draft is kept: everything you changed up to the last saved change is still there.
My image was not accepted.
Images must be PNG, JPG or GIF files, up to 1 MB, as on Configure Theme. Save the image in one of these formats, or make it smaller. If even small images are refused, the server's upload limit (upload_max_filesize and post_max_size in PHP's settings) may be lower than 1 MB.
The preview shows a page without my changes.
The page left preview mode, for example after following a link that a script built without the preview parameter. Click Close, then open the customizer again, or use the browser's Back button inside the preview.
An exhibit with a theme of its own (set in Exhibit Builder) always shows that theme's settings; a note above the preview says so.
The preview area stays blank.
The preview shows your site in a frame. If your web server or a security plugin sends X-Frame-Options: DENY or Content-Security-Policy: frame-ancestors 'none' on public pages, browsers refuse to show them in a frame, even on the same site. Allow same-origin framing instead (X-Frame-Options: SAMEORIGIN or frame-ancestors 'self'). Open in new tab still works meanwhile.
Can two people customize the same site at once?
Yes. Each person has their own draft, holding only the settings they changed. Publishing applies those settings; settings someone else changed and published stay as they are, unless you changed the same setting.