aseprite/data/widgets/timeline_conf.xml

<!-- ASEPRITE -->
<!-- Copyright (C) 2014, 2015 by David Capello -->
<gui>
<vbox id="timeline_conf">
  <separator cell_hspan="2" text="Onion Skin:" left="true" horizontal="true" />
  <grid columns="2">
    <hbox cell_hspan="2">
      <radio group="1" text="Merge Frames" id="merge" />
      <radio group="1" text="Red/Blue Tint" id="tint" />
      <button id="reset_onionskin" text="Reset" width="60" />
    </hbox>

    <label text="Opacity:" />
    <slider min="0" max="255" id="opacity" cell_align="horizontal" width="128" />

    <label text="Opacity Step:" />
    <slider min="0" max="255" id="opacity_step" cell_align="horizontal" width="128" />

    <check id="loop_tag" text="Loop through tag frames" cell_hspan="2" />
    <check id="current_layer" text="Current layer only" cell_hspan="2" />
    <hbox cell_hspan="2">
      <radio group="2" text="Behind sprite" id="behind" tooltip="Only for transparent layers.&#10;Background is not included in this onion skin mode." tooltip_dir="top" />
      <radio group="2" text="In front of sprite" id="infront" tooltip="For all kind of layers (background and transparents)" />
    </hbox>
  </grid>
</vbox>
</gui>