Well, window button icons are defined through PNGs in unity and metacity-1 folders in every theme, so replacing those PNGs easily results in changing the icons. If you are interested, I'll share my experience here.

You can find the default ones for Ambiance theme here:

/usr/share/themes/Ambiance/unity/

/usr/share/themes/Ambiance/metacity-1/

Since there are different modes for a window button, such as prelight, pressed, focused, you should have different shades for a single button; for example there are 7 PNGs in unity folder for different modes of minimize button. So I used GIMP to edit the default icons and make a new ones with a ‹ – Single Left-Pointing Angle Quotation Mark – and here's the result: .

Now, if you want to change the minimize button to a single left-pointing arrow like this, download the PNGs created for Ambiance and Radiance themes here. Then extract the file and follow the procedure in Read Me file. I'm going to repeat it here:
Open a terminal (Ctrl+Alt+T) and run these two commands (let assume that the extracted file is in Downloads directory):

cd Downloads/minimize-button



sudo cp -r Ambiance Radiance /usr/share/themes

(edit) If you don't want to change system files, or you are a user in a shared desktop and thus can't/don't want affect other users, you can download this file instead. After you will extract the file, put two folders Ambiance and Radiance (or each you want) under the path:

~/.themes/

– if it doesn't exist, simply create it ("~" stands for your home folder as you may know).

This alternative doesn't need a root access.

SVG’s for Unity 7 (14.04)

Supporting displays with higher resolutions, Unity 7 have SVG’s for window buttons. This file includes SVG’s of arrow minimize buttons for both Ambiance and Radiance. It includes buttons for Dash window too.