Am constantly frustrated by the silly modish fad to remove lines separating toolbars and other items such as groups of headings; they are important visual signals. Lots of icons floating in space may work for those brought up on iFads; they do not work for me! The key to understanding minimalism is that less is less.
I have not yet tried it, but you can choose your own theme in Tools > Options > LibreOffice > Personalization. A Firefox theme of category “classic” might work for you. Here is a video HowTo http://flosmind.wordpress.com/2013/01/18/personas-in-libreoffice-color-your-office/