As of v15.0 Webmin uses a new default theme called "Authentic". It provides a much more modern interface (based on Bootstrap) but otherwise should server the same purpose. It does use a few more resources, so users on lower resource servers may wish to revert to the previous theme, "Stressfree".

Change the default Authentic Theme page to Dashboard

By default Webmin on TurnKey will default to the TKLBAM module page. This can be easily adjusted to anything you prefer. I'll assume that many users will want to set the "Dashboard" as the default. However, using these instructions it should be fairly obvious on how to set any of the other modules as default.

Select the "three cogs" icon from the bottom of the left pane. When you hover over it, it says "Theme configuration".

In the main pane (right hand side) click "Default pages" (towards the top, in the middle, right above "Global options). Next to "Default module for Webmin" there is a dropdown with "Backup and Migration (TKLBAM)" currently showing. Click on the drop down. To make the "Dashboard" the default landing page, simply select the (top most) empty option (i.e. so there is no default module). If you'd rather another module to be the default, select it instead.

Then scroll down to the bottom of the page and click the "Save" button. You should now be done. To test it, log out and then log back in.

Revert to legacy Stressfree theme

From the commandline:

apt update
apt install webmin-theme-stressfree
apt purge webmin-authentic-theme
sed -i "s|authentic-theme|theme-stressfree|" /etc/webmin/config
service webmin restart

Note: this should also be possible from within Webmin itself, but I haven't tested that. If someone would like to update these docs to include that, please feel free (they are a wiki and all logged in users should be able to edit them).