If your change appears in the editor but not where you normally read Tumblr, check the address. A custom theme applies to the blog's blogname.tumblr.com view; the dashboard's blog view is a different surface. Theme editing is currently available on the web. Custom-theme availability and views
Save the current theme before changing it
Copy the existing source into a dated local text file before the first edit. Keep the copy associated with the correct blog. If you manage a personal blog and a project blog, check the selected name before replacing anything.
For a first change, pick one visible phrase or link you can locate in the source. This is easier to inspect than replacing a whole theme and then trying to identify why several sections changed.
The following is an original illustrative navigation fragment, not a complete Tumblr theme:
<nav aria-label="Blog navigation">
<a href="/">Home</a>
<a href="/about">About this project</a>
</nav>
If the actual About page uses another URL, use that URL. Do not add a link to /about simply because the example does. Keep Tumblr's existing theme variables and post blocks intact while changing the link you intended to edit.
Preview the change in the right context
Click Update Preview and find the changed phrase or link in the adjacent theme preview. Check that nearby content still appears. Then save through the Customizer and open the blog's custom-theme URL to inspect the result.
For the navigation example, the useful checks are concrete: the link text reads “About this project,” the destination opens the actual About page, and the navigation remains readable at a narrow width. A preview with the right color alone does not verify the destination.
If the blog redirects to tumblr.com/yourname, review whether Custom theme is enabled. Visibility settings can also affect access. Keep your intended audience in mind before changing a setting that hides the blog from people without an account. Theme view and visibility
Recover a bad edit before making more changes
If an edit produces a blank or broken layout, restore your saved source and narrow the change. Look near the edit for an unclosed tag, missing bracket, or damaged theme variable. When saving fails, Tumblr's troubleshooting advice includes checking for HTTP links that need HTTPS. Inspect the specific URL before replacing text broadly. HTML troubleshooting
Tumblr also keeps recent custom-HTML versions on its Theme Recovery page. Choose the blog and inspect the available backup before reverting. Recent history is useful, but a version too many revisions back may no longer be available. Keep your own known-good copy. Recovery limits
For a separate full HTML page, saving a self-contained HTML file is a different task. A Tumblr theme depends on Tumblr's template system. Finish this edit by checking the actual blog view and its links, then keep the restored or corrected source with a note describing what changed.