Set text color in styled cluster highlights

This gives us two assurances:

1. Highlight background colors, if not amazing on dark backgrounds in
   HTML documents, are at least readable
2. We know the exact contrast between foreground and background text in
   highlight elements —— think accessibility here in the future

src/styles/annotator/highlights.scss

@@ -4,6 +4,7 @@
   --hypothesis-highlight-focused-color: rgba(156, 230, 255, 0.5);
   --hypothesis-highlight-blend-mode: normal;
   --hypothesis-highlight-decoration: none;
+  --hypothesis-highlight-text-color: inherit;
 
   --hypothesis-highlight-second-color: rgba(206, 206, 60, 0.4);
   --hypothesis-highlight-third-color: transparent;
@@ -20,6 +21,7 @@
   // Clustered highlight styling configuration
   // These values are updated by the `highlight-clusters` module
   --hypothesis-cluster-blend-mode: multiply;
+  --hypothesis-cluster-text-color: #333333;
   --hypothesis-other-content-color: var(--hypothesis-color-yellow);
   --hypothesis-other-content-decoration: none;
 
@@ -35,6 +37,7 @@
 .hypothesis-highlight,
 .hypothesis-svg-highlight {
   --highlight-color: var(--hypothesis-highlight-color);
+  --highlight-text-color: var(--hypothesis-highlight-text-color);
   --highlight-blend-mode: var(--hypothesis-highlight-blend-mode);
   --highlight-decoration: var(--hypothesis-highlight-decoration);
   --highlight-color-focused: var(--hypothesis-highlight-focused-color);
@@ -51,6 +54,11 @@
 
 // Configure clustered highlight styling. The `.hypothesis-highlights-clustered`
 // class is managed by `highlight-clusters`
+
+.hypothesis-highlights-clustered .hypothesis-highlight {
+  --highlight-text-color: var(--hypothesis-cluster-text-color);
+}
+
 .hypothesis-highlights-clustered .hypothesis-highlight,
 .hypothesis-highlights-clustered .hypothesis-svg-highlight {
   // When clustered highlights are active, use an opaque blue for focused
@@ -127,6 +135,7 @@
 }
 
 .hypothesis-highlights-always-on .hypothesis-highlight {
+  color: var(--highlight-text-color);
   background-color: var(--highlight-color);
   text-decoration: var(--highlight-decoration);
   mix-blend-mode: var(--highlight-blend-mode);