You can also change the manager_theme System Setting value and write your own Manager theme.
You could duplicate everything in the manager/templates/default to something like a manager/templates/mytemplate directory, set the manager_theme System Setting to mytemplate, and rewrite whatever you want in the mytemplate directory.
I've never done it and it looks like a pretty big job.
A more limited way is to just create that System Setting with its new value, then copy the files you want to change to the mytemplate directory and change just those files. MODX will use the new files but will revert to the default files if they're not found in your directory. This way will slow the Manager down a little because it's constantly looking in the wrong directory first for most of the files, so I'd be inclined to use the first method and just change individual files as needed.
Note that some extras change the manager_default setting (mostly custom Manager theme packages).
There are some more custom themes available here: https://www.google.com/search?q=modx+manager+theme&oq=modx+manager+theme
, but I don't know which ones will work with the current version of MODX.