* Added "Arc Dark" theme with blue and orange accented colors.
* Added light and dark editor color schemes. Light matches
  IntelliJ Light and Dark overrides Darcula with many custom color,
  since originally it was too dark for the panel/editor and text
  was not clearly visible.
* Unified hover effect on buttons and tabs to be lighter and borders
  on button to better match real Arc theme.
* Unified tabs styling, have only hover effect, keep panel color
  on selected tab (except editor tabs).