Add more color documentation, Add light buttons to docs

docs/bulma-dark.sass

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+@import ../sass/utilities/initial-variables.sass
+
+// General colors
+
+$scheme-main: $black
+$scheme-main-bis: $black-bis
+$scheme-main-ter: $black-ter
+$scheme-invert: $white
+$scheme-invert-bis: $white-bis
+$scheme-invert-ter: $white-ter
+
+$background: $black-ter
+
+$border: $grey-darker
+$border-hover: $grey-dark
+$border-light: $grey-darker
+$border-light-hover: $grey-dark
+
+// Text colors
+
+$text: $grey-light
+$text-invert: $grey-darker
+$text-light: $grey
+$text-strong: $white
+
+// Link colors
+
+$link-hover: $white
+$link-hover-border: $grey-dark
+
+$link-focus: $white
+$link-focus-border: $blue
+
+$link-active: $white
+$link-active-border: $grey-light
+
+.bd-navbar
+  background-color: $scheme-main-bis
+
+  &.has-shadow
+    box-shadow: none