Select the menu item Preferences/User File Preferences, then add the line:
font consolas 10
and save the file. This will change the font 10pt Consolas. You can also add any of these suffixes:
font courier new 10 :noantialias
will change the font to 10pt Courier New, with anti-aliasing disabled.
You can also edit many other options in the same way: have a look around Preferences/Default File Preferences to see what's available.
Most graphical issues, such as the window appearing black, or flickering when opening menus, can be resolved by running in OpenGL mode. There are two ways to do this:
Yes. Sublime Text may be installed to any location, however it will read and write your settings to %APPDATA%\Sublime Text. You can change this to use a directory on the USB key, by running Sublime Text with the --data flag, for example:
SublimeText.exe --data "E:\Sublime Text Data"