correct colors on Threads screen

Refs #229, #226, #218

This adds style overrides for the new selectors Slack begain using for
its Threads screen, beginning with the .p-threads_view container.

Test Plan:
- Have one or more threads that you've participated in and view the
  "Threads" screen from the left navigation panel
- Verify that the colors match the theme and everything is legible
- Verify that the button links for loding more replies in a thread are
  colored to match the theme and are legible

scss/modules/messaging/_threads.scss

@@ -45,6 +45,45 @@
   background: $color-base;
 }
 
+.p-threads_view {
+  background: $color-shade-dark;
+
+  &__divider {
+    &_line,
+    &_label {
+      background: $color-shade-darker;
+      border-color: $color-shade-darker;
+      color: $base-font-color;
+    }
+  }
+
+  &__default_background {
+    background: $color-shade-darker;
+  }
+
+  &_header {
+    &__channel_name,
+    &__participant_list {
+      color: $base-font-color;
+    }
+  }
+
+  .p_threads_view_load_newer_button,
+  .p_threads_view_load_older_button {
+    background: $color-shade-darker;
+
+    .c-link--button {
+      color: $base-link-color;
+    }
+  }
+
+  &_reply {
+    &--new_reply {
+      background: $color-shade-darker;
+    }
+  }
+}
+
 .p-threads_footer__input,
 .p-threads_footer__input--legacy {
   .p-message_input_field {