Add white black light dark colors, Fix button inverted

sass/elements/other.sass

@@ -2,7 +2,7 @@
   +unselectable
   -moz-appearance: none
   -webkit-appearance: none
-  background: rgba(black, 0.2)
+  background: rgba($black, 0.2)
   border: none
   border-radius: 290486px
   cursor: pointer
@@ -13,7 +13,7 @@
   width: 24px
   &:before,
   &:after
-    background: white
+    background: $white
     content: ""
     display: block
     height: 2px
@@ -28,7 +28,7 @@
   &:after
     transform: rotate(-45deg)
   &:hover
-    background: rgba(black, 0.5)
+    background: rgba($black, 0.5)
   // Sizes
   &.is-small
     height: 16px
@@ -160,9 +160,6 @@
     &.is-#{$name}
       background: $color
       color: $color-invert
-  &.is-dark
-    background: $text
-    color: $text-invert
   // Sizes
   &.is-small
     font-size: $size-small