Notice: this information was written when Microthemer version 2 was current. On Dec 26th, 2014 version 3 was released with a completely new interface. Some of the information may be redundant, but as much of
the functionality remains the same in version 3 we haven't deleted this post.
Unless other properties, Line Height does not assume px by default. The unit of measurement has to be entered or the rendered page goes all to pot.
Yes this is deliverate actually. You don’t need to specify a unit value for CSS line-height. If the font size is 10px and you specify a line-height of 1.5 the result will be the same as specifying a line-height of 15px. I think the former option is better in most circumstances (to keep to spacing proportional to the font-size).
But the main reason Microthemer doesn’t default to pixels even though that would be consistent with other settings is that, if it did, it wouldn’t be possible to override the funcitonality and set only a number.