I am using Linux Mint with Cinnamon. Whenever I install a theme, it changes the 'system' elements, like the menu and taskbar, but not the window borders or buttons or the like. I went on the 'Other Settings' tab in the 'Themes' window, but no options other than the ones that came with the system appear. Specifically, I want the Holo theme to be system-wide.
This is sort of a partial answer since I'm not a user of Mint or Cinnamon.
The general appearance of the GUI in linux is mostly determined by two things:
The theme of the widget library (aka. toolkit), which is usually primarily GTK. "Primarily", because in fact the toolkit used by an application is determined by the application itself, but the most common one is GTK. The DE (desktop environment) includes a bunch of applications of its own and these will all use the same toolkit; Cinnamon is a GTK based DE. However, keep in mind that it's possible to run applications which don't use GTK, in which case, they will probably not match the theme.
The widget library is what's responsible for the stuff inside the window borders. There are various ways to change this, DE's will include one (which you've evidently found).
The theme used by the window manager. The window manager is a layer underneath the DE, and it determines how windows look and behave. They employ themes too, but these are unrelated to any widget theme. Unlike the widget toolkit, the window is not determined per application, and so the border, titlebar, etc. will appear the same regardless.
DE's usually have a way to change the window manager theme too, but it will probably be separate from the widget theme. On KDE, for example, #1 is the "Desktop Theme" and #2 is the "Window Decorations". So you want to look for a distinction of that sort; a quick online search for "linux mint cinnamon change window theme" turned up this, which mentions the window borders, etc., in contrast to the GTK theme.