Austin, you may have went a little overboard
These posts don't really have much room for discussion. That said I just chose one. After all I was the one who said we would come.'
When Theming the Main Menu, One should first and foremost Chose a good graphic for the background. This goes for quite a few of these How to's on the subject of theming. Yes you can just go minimalist and use a solid color or a gradient, but graphically speaking its not very interesting to look at.
A "good" graphic should be Large. Wallpaper size is good, HD is better. But for just a single element, A good image is one that is larger then the element you are theming. Lets use this one for now http://upload.wikimedia.org/wikipedia/commons/b/b0/Mint-leaves-2007.jpg
Generally speaking the Main Menu Element is usually around 638x634. So take that image into gimp scale it to a height of 634 so that you can get the majority of the image in without it scaling badly when its applied to your main menu. Set the canvas size to a width of 638 and center the image, then set the layer to image size.
Now you have a nice menu sized image for your main menu. But lets add a few extra touches to your graphic to make it pop. I like to bevel mine so using Filters/Decor/Add Bevel in gimp, Uncheck work on copy, set the bevel to 20 and apply. then hit [Ctrl]+[F] to do it again (not necessary on all images)
Save this image to your Desktop As Menu.Png. Move this image into the theme you are modding, Lets use Cloud 1 http://cinnamon-spices.linuxmint.com/themes/view/72
. which should be already in your .themes folder. if not download it from the supplied link.
Open the cloud 1 cinnamon.css file in Gedit.. go to the PopupMenu (popupMenu.js) section. in the subsection listed as .popup-menu, replace the line
background-color: rgba(255,255,255,0.3); with border-image: url("Menu.png")0 10 8 8 stretch;
save the cinnamon.css and apply the cloud 1 theme and restart cinnamon