Dark theme, theme switch, icons, style, strings

added some white icons,
changed hardcoded icons to theme attributes,
changed icon_edit_dark to icon_edit_body to reflect icons position,
grey message bubbles in dark theme,
misc

purged ic_action_chat as it wasn't used

preference use_white_background changed to use_green_background, default true

grey chat bubbles darker, text white

replaced all grey600 with black icons and 0.54 alpha attribute

highlightColor in dark grey chat bubble now darker than background