guys, Please IGNORE this reply, there is something wrong with it.
xev didn’t work for my mouse, I used xinput to find out the device ID of my mouse first, and then use "xinput test 12 (mouse ID) to find out that the side buttons are actually button 8 and 9.
I have previously installed imwheel to adjust mouse scrolling speed already, I found the script in other forum and added it to startup application.
http://www.nicknorton.net/mousewheel.sh
i simply added these 2 lines to the script.
None, Button8, Alt_L|KP_Left
None, Button9, Alt_L|KP_Right
it took me a while to figure out the Alt_L|KP_left/right bits, though. Most of the examples in imwheel file are not applicable in this case, surprisingly these buttons now work in other programs as well!
i hope it is helpful for you guys as well.