Clean overridable property in settings, and change installed app order to ensure the correct template overriding
.diff-viewer-wrapper {margin-top: 5px;}
.diff-panel {border: 1px solid gray; width: 300px; heigth: 250px;}
del { text-decoration: none; color: #b30000; background: #fadad7;}
ins { background: #eaf2c2; color: #406619; text-decoration: none; }