Add color variables of texts for better accesibility (#7125)

* Add variables for text colors

* Change variables in sass files

* Apply text color variables for recently added colors

* Fix text colors of emoji mart anchors

* Fix text colors of search__input

* Fix text colors of text area of compose-form

* Fix icon colors of privacy dropdown and modal

* Inverted icon colors by classname

* Change variables in boost.scss

* Change action-button-color

* Fix text colors of pre-header

app/javascript/styles/mastodon/basics.scss

@@ -75,7 +75,7 @@ body {
   &.error {
     position: absolute;
     text-align: center;
-    color: $ui-primary-color;
+    color: $darker-text-color;
     background: $ui-base-color;
     width: 100%;
     height: 100%;