Change default framework view (issue #17)

This version introduces multiple enhancements:

applied bootstrap_dark for all pages in tree.xml
some small changes inside bootstrap_dark

Details of additions/deletions below:
--------------------------------------------------------------

Modified:   /BLUE_FRAMEWORK/elements/css/styles.css
            -- Removed --
                background for body element
Modified:   /BLUE_FRAMEWORK/cfg/tree.xml
            -- Added --
                bootstrap_dark for all pages
            -- Removed --
                bootstrap_dark from examples and commit
Modified:   /BLUE_FRAMEWORK/elements/css/bootstrap_dark.css
            -- Updated --
                changed font and background color for pre element