New checkbox look for dark theme

ApplicationCode/Resources/ResInsight.qrc

@@ -203,6 +203,8 @@
         <file>themes/dark/expanded.svg</file>
         <file>themes/dark/arrow-down.svg</file>
         <file>themes/dark/arrow-up.svg</file>
+        <file>themes/dark/checkbox-unchecked-20x20.png</file>
+        <file>themes/dark/checkbox-checked-20x20.png</file>
         <file>SummaryCase24x24.png</file>
     </qresource>
     <qresource prefix="/Shader">

ApplicationCode/Resources/themes/dark.qss

@@ -24,6 +24,18 @@ $plotGridColor: #394046; // Plot grid color
 $auxiliaryCurveColor: #000000; // Auxiliary curve color
 $externalInputColor: #e82257; // External input background color
 
+QCheckBox::indicator:unchecked
+{
+    background-color: $backgroundColor1;
+    image: url(:/themes/dark/checkbox-unchecked-20x20.png);
+}
+
+QCheckBox::indicator:checked
+{
+    background-color: $backgroundColor1;
+    image: url(:/themes/dark/checkbox-checked-20x20.png);
+}
+
 QwtPlot["*"]::grid["*"] {
     color: $backgroundColor2;
 }
@@ -151,6 +163,18 @@ QTreeView::branch:open:has-children:has-siblings {
     icon-size: 12px;
 }
 
+QTreeView::indicator:unchecked
+{
+    background-color: $backgroundColor1;
+    image: url(:/themes/dark/checkbox-unchecked-20x20.png);
+}
+
+QTreeView::indicator:checked
+{
+    background-color: $backgroundColor1;
+    image: url(:/themes/dark/checkbox-checked-20x20.png);
+}
+
 QDockWidget {
     background-color: $mainBackgroundColor;
     titlebar-close-icon:  url(:/themes/dark/blank.svg);
@@ -370,4 +394,4 @@ QTextEdit, QPlainTextEdit {
 QWidget[state="ExternalInput"] {
     background-color: $externalInputColor;
     border: 2px solid $borderColor;
-}
+}