changed some colors for better readability

[michiwend]

• lighter color columns. It irritated me when the color-column was darker then window split lines. Now they are the same color (like in solarized theme)
• Grey change signes for GitGutter are nice and discreet but useless to me. I could not see my changes at first sight. Now they use the pre-defined diff colors.
• Added light grey bg to folded lines (maybe only useful to me as i do not have textual fold lines like "-----")