md-select bottom-border is black when focused after commit # 8468c60

[TalPasi]

Since the default theme is removed from the selectors in commit # 8468c60,

The generated default-theme rule for the border-bottom of md-select-label is overridden by the following rule:

md-select:not([disabled]):focus .md-select-label {
  border-bottom: 2px solid ;
}

The border-bottom gets the initial color of black since no color was provided in the shorthand value.