Cursor themes

Emerge
Check if you already have the cursors you need before installing any new package. Cursors can be found in the directory.

comes with the whiteglass and the redglass themes:

Additional software
Another package with some Gentoo themed cursors is available as well

comes with gentoo, gentoo-blue, and gentoo-silver cursor themes:

To install all of the cursor themes available from Portage:

Configuration
Edit your file:

Cursor size can be optionally chosen as well:

Cursor keeps going back to default X cursors
Some desktop environment, mouse theme changes when hovering otherside of window. Inside the window, the mouse cursor is set what I want. But when go outside of the window(such as i3-bar, i3-status, background, etc), the mouse cursor changes into default X cursor.

Create the symlink to your home path.

GTK2/GTK3
Some GTK application has the problem which the cursors change to default X cursors when the cursor is in the GTK application. Go to settings.ini and modify the value. Change gtk-theme-name and gtk-cursor-theme-name to what you want. On this example below, we choose 'gentoo'.

External resources

 * Article on the Arch Linux wiki