ssh-askpass(1) is a graphical program that prompts the user for their SSH Key passphrase and passes it to the ssh-add(1) command.
Out of the box, ssh-askpass is pretty ugly, but I feel like I've managed to make it look dark, modern, and good.
A screenshot
This is how my ssh-askpass window looks:
The code
SshAskpass*Dialog.font: -adobe-helvetica-bold-r-normal-*-*-120-*-*-*-*-iso8859-1
SshAskpass*Dialog.label: Please enter your authentication passphrase:
SshAskpass*Dialog.title:
SshAskpass*background: #000000
SshAskpass*foreground: #cccccc
SshAskpass.Button*background: #000000
SshAskpass.Button*foreground: #cccccc
The above code goes in your ${HOME}/.Xresources file.
On Arch Linux, make sure you have the xorg-xrdb package installed.
I have a line in my ${HOME}/.xprofile that reads:
[ -r $HOME/.Xresources ] && xrdb -merge -I$HOME $HOME/.Xresources
Stuff like this isn't super important, but when you're typing your SSH key passphrase 20 times a day, it's nice when you like the look of the dialog box.