Casing consistency of theme properties

[lionel-]

Most properties are dash-cased except these ones which are snake_cased: vec_last, vec_sep, vec_trunc, and string_quote. Should the latter be superseded in favour of dash-cased properties?

[lionel-]

Maybe also replace all dashes by underscores in property values, so that color = "br-blue" is equivalent to color = "br_blue"?

[gaborcsardi]

Many style names are coming from CSS, so I will rather change the underscores into dashes.