\fbox inherits default color (#1847)

Merge `.fbox` and `.fcolorbox` rules to inherit default color, instead
of forcing `black`.  We no longer have any explicit `black` in CSS.
Fix #1772.

src/katex.less

@@ -542,11 +542,7 @@
         padding: 0 0.3em 0 0.3em;      // \fboxsep = 3pt
     }
 
-    .fbox {
-        box-sizing: border-box;
-        border: 0.04em solid black;   // \fboxrule = 0.4pt
-    }
-
+    .fbox,
     .fcolorbox {
         box-sizing: border-box;
         border: 0.04em solid;         // \fboxrule = 0.4pt