Look at the second to last section of the Formatting cheatsheet. It covers indentation, [@Technonick].

Its not quite as sparse as whitespaces, but it looks good.
Alternatively, you can use colored underscores. The hex color you want is 2e2c2c.

see examples here:
indent:
[indent]a[/indent]
[indent][indent]a[/indent][/indent]
[indent]a[/indent]
[indent][indent]a[/indent][/indent]
[indent][indent][indent]a[/indent][/indent][/indent]

colored underscore:
a
[color=2e2c2c]_[/color]a
[color=2e2c2c]_[/color]a
[color=2e2c2c]__[/color]a
[color=2e2c2c]__[/color]a
[color=2e2c2c]_[/color]a
[color=2e2c2c]__[/color]a
[color=2e2c2c]__[/color]a
[color=2e2c2c]___[/color]a