Fix background of lower corners in tool palette

The tool palette has somewhat ugly differently colored
pixels in the lower left and right corner as a
different background is inherited and shines
through because of the rounded corners.
With this change, we unset the background to get
rid of this effect.

eclipse-glsp/glsp#492

Change-Id: Ibe0c331c44d74f2369549fd2ab966e8135542154

packages/theia-integration/css/tool-palette.css

@@ -20,6 +20,10 @@
     width: 210px;
 }
 
+.tool-group {
+    background-color: unset;
+}
+
 .tool-palette .palette-header {
     background: var(--theia-titleBar-activeBackground);
     color: var(--theia-titleBar-activeForeground);