update for breaking blueprintjs style changes

it is no longer feasible to build the blueprintjs css ourselves, so we
just use all of the default variable values and override things using
selectors instead.

src/editor/editor.scss

@@ -3,7 +3,7 @@
 
 // Custom styling for the Editor control.
 
-@import '../variables.scss';
+@use '@blueprintjs/core/lib/scss/variables' as bp;
 
 // add "BETA" watermark
 
@@ -19,8 +19,8 @@
     pointer-events: none;
 }
 
-.#{$ns}-dark .pb-editor-placeholder {
-    color: $pt-dark-text-color-muted;
+.#{bp.$ns}-dark .pb-editor-placeholder {
+    color: bp.$pt-dark-text-color-muted;
 }
 
 .pb-editor-tab {
@@ -34,7 +34,7 @@
 .pb-editor-placeholder {
     pointer-events: none;
     width: max-content;
-    color: $pt-text-color-muted;
+    color: bp.$pt-text-color-muted;
     font-style: italic;
     padding-left: 4px;
 }