[DiapDealer]

The easiest way on Linux is to use qt5ct and/or kvantum to pick a qt style for Sigil that you like.

If you don't want to do that, you can create a custom qt style file for Sigil and use Qt's qss (much like css) to style individual elements. Create a file named qt_styles.qss in your Sigil preferences folder and it will automatically be loaded whenever Sigil starts. Something like below will allow you to modify the various colors of ALL QTreeView objects in Sigil:

QTreeView {
  alternate-background-color: blue;
  background: yellow;
  selection-color: white;
  selection-background-color: red;
}

You can limit it to the Clip Editor's TreeView with:

QTreeView#ClipEditorTree {
  alternate-background-color: blue;
  background: yellow;
  selection-color: white;
  selection-background-color: red;
}