For High Contract Themes the decoration is a double border under the enlargement.
When using command Preferences: Color Theme (ctrl+k ctrl+t, workbench.action.selectTheme) the settings.json file is not updated with the new theme name when you use the Up/Down Arrow keys. The decorator style is not changed when you switch between Ligh/Dark and High Contrast themes. When you select a theme the decorator style is changed.
The reason is that for the current version of VSC (v1.74) it is not possible to define a decorator that has different styles for Light/Dark and High Contrast Themes. A High Contrast Theme uses the light or dark settings of the decorator.
indentWhitespace.space.enlargement : number ∈ ℕ (>= 0), the number of spaces to enlarge each indent space, if 0 there is no decoration, (default: 1)
indentWhitespace.space.positionDecorator : [ "before" | "after" ] Position of the indent space decorator, (default: "after")
The extension defines the following Theme Color:
indentWhitespace.space.enlargement : Color for the indent space decoration.