Do you wish to customize the look and feel of the stock Messages app on your iPhone? If so, you might be interested in a jailbreak tweak known as MessagesCustomiser by iOS Developer Charlie Hewitt.
What I like mostly about this tweak is that it features a reset button so that you can revert back to the default settings in case you mess up with the configuration. Once you’re done with the customization, all you have to do is to close the Messages app and launch it again for the new changes to take place.
MessagesCustomiser is currently in the beta phase so you might run into bugs while using it. For instance, when you toggle on the gradient option for message bubbles, it crashes the Settings app after getting enabled.
If you’re interested to give this tweak a try, you’ll have to add the following new source to Cydia first: http://chewitt.me/repo/ . Once done, search for the tweak and install it on your device.
Let us know how the tweak performs on your iOS 8 device.