Default line_percent_gradient to (light|dark)_green_gray gradients

dark_green_gray gradient was generated with

    tools/generate_gradients.py 70 247

. It is used for non-insert modes in the current window. light_green_gray
gradient was generated with

    ./tools/generate_gradients.py 148 250

. It is used for insert mode in the current window.
Merge pull request #915 from ZyX-I/better-line_percent_gradient

Default line_percent_gradient to (light|dark)_green_gray gradients