message: Add locale texts for the "Re-enable Dark Theme" setting and translate it into Japanese
diff: <li data-setting-name="og_dark_theme">
  <input type="checkbox" name="css.og_dark_theme" id="og_dark_theme">
  <label for="og_dark_theme" data-lang="css_og_dark_theme">Re-enable the Dark theme (pre February 2018)</label>
  - <small>This is experimental and can break at any point!</small>
  + <small data-lang="css_experimental_warning">This is experimental and can break at any point!</small>
  </li>
  <li data-setting-name="no_col_icns">
  <input type="checkbox" name="css.no_col_icns" id="no_col_icns">
  