You can easily customize many options for the GNOME desktop by opening System > Preferences > Appearance. However, these customizations are somehow limited because you are not allowed to customize for example the color of some specific buttons or color of scrollbars that are displayed on the right side of an application or text document.
Fortunately, there is a small application in the official Ubuntu repository that will meet your expectations. It is called GNOME Color Chooser which allows to customize GNOME's global colors, buttons, panel, Desktop, scrollbars, windows, progress bars, links, etc.
After selecting desired colors, changes to the appearance of your system will be applied instantly after clicking the Apply button. However, changes might not be applied immediately due to the current used theme, you can change it via System > Preferences > Appearance.
GNOME Color Chooser Installation
Launch the terminal and issue this command:
sudo apt-get install gnome-color-chooser
You can also install it using this link. To start the application, open System > Preferences > GNOME Color Chooser.