Use border radius CSS var for other things too.

packages/application/style/buttons.css

@@ -15,7 +15,7 @@
 
 
 button {
-    border-radius: var(--jp-button-border-radius);
+    border-radius: var(--jp-border-radius);
 }
 
 

packages/cells/style/collapser.css

@@ -11,7 +11,7 @@
   border: none;
   outline: none;
   background: transparent;
-  border-radius: var(--jp-button-border-radius);
+  border-radius: var(--jp-border-radius);
   opacity: 1.0;
 }
 

packages/cells/style/placeholder.css

@@ -33,7 +33,7 @@
   width: 32px;
   height: 16px;
   border: 1px solid transparent;
-  border-radius: var(--jp-button-border-radius);
+  border-radius: var(--jp-border-radius);
 }
 
 .jp-Placeholder-content .jp-MoreHorizIcon:hover {

packages/theme-dark-extension/style/variables.css

@@ -37,7 +37,7 @@ all of MD as it is not optimized for dense, information rich UIs.
   --jp-border-color1: var(--md-grey-800);
   --jp-border-color2: var(--md-grey-800);
   --jp-border-color3: var(--md-grey-900);
-  --jp-button-border-radius: 2px;
+  --jp-border-radius: 2px;
 
   /* UI Fonts
    *

packages/theme-light-extension/style/variables.css

@@ -37,7 +37,7 @@ all of MD as it is not optimized for dense, information rich UIs.
   --jp-border-color1: var(--md-grey-400);
   --jp-border-color2: var(--md-grey-300);
   --jp-border-color3: var(--md-grey-200);
-  --jp-button-border-radius: 2px;
+  --jp-border-radius: 2px;
 
   /* UI Fonts
    *