Improve border parsing in themes

The `top`, `left`, `bottom` and `right` fields are optional.
If none are specified, then they are all set to true.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
d37a98782eMax Brunsfeld committed on 8/5/2021, 12:19:42 AMparent522fac9
4 files changedLine totals unavailable