Use outline only indicator icon

images/images.qrc

@@ -709,6 +709,7 @@
         <file>themes/default/mDockify.svg</file>
         <file>themes/default/mActionReverseLine.svg</file>
         <file>themes/default/mActionAdd3DMap.svg</file>
+        <file>themes/default/mIndicatorNonRemovable.svg</file>
     </qresource>
     <qresource prefix="/images/tips">
         <file alias="symbol_levels.png">qgis_tips/symbol_levels.png</file>

images/themes/default/mIndicatorNonRemovable.svg

@@ -0,0 +1,72 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   height="16"
+   width="16"
+   version="1.1"
+   id="svg8"
+   sodipodi:docname="mIndicatorLocked.svg"
+   inkscape:version="0.92.3 (2405546, 2018-03-11)">
+  <metadata
+     id="metadata14">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs12" />
+  <sodipodi:namedview
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1"
+     objecttolerance="10"
+     gridtolerance="10"
+     guidetolerance="10"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:window-width="3200"
+     inkscape:window-height="1633"
+     id="namedview10"
+     showgrid="false"
+     showguides="true"
+     inkscape:guide-bbox="true"
+     inkscape:zoom="29.5"
+     inkscape:cx="17.357518"
+     inkscape:cy="6.7658225"
+     inkscape:window-x="0"
+     inkscape:window-y="0"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg8" />
+  <path
+     style="fill:none;stroke:#2c2c2c;stroke-width:3;stroke-linecap:butt;stroke-linejoin:round;stroke-opacity:0.70588235"
+     inkscape:connector-curvature="0"
+     d="M 4.474418,7.9 C 4.592418,4.082 5.4064688,1.769 8.2774688,2.002 11.147468,2.234 11.657875,4.082 11.537875,7.9"
+     id="path2"
+     sodipodi:nodetypes="ccc" />
+  <path
+     style="fill:none;stroke:#ffffff;stroke-linecap:butt;stroke-linejoin:round;stroke-opacity:1"
+     inkscape:connector-curvature="0"
+     d="m 4.3264688,9.451 0.116,-2.135 c 0.12,-3.818 0.95,-5.59 3.82,-5.358 C 11.132468,2.19 11.778468,4.081 11.659468,7.9 l -0.085,1.551"
+     id="path4" />
+  <rect
+     y="8.2999992"
+     x="2.5"
+     width="11"
+     ry="1"
+     rx="0.84600002"
+     height="7"
+     id="rect6"
+     style="fill:#ffffff;stroke:#2c2c2c;stroke-linecap:square;stroke-opacity:0.70588235;fill-opacity:1" />
+</svg>

src/app/qgslayertreeviewnonremovableindicator.cpp

@@ -24,7 +24,7 @@ QgsLayerTreeViewNonRemovableIndicatorProvider::QgsLayerTreeViewNonRemovableIndic
   : QObject( view )
   , mLayerTreeView( view )
 {
-  mIcon = QgsApplication::getThemeIcon( QStringLiteral( "/lockedGray.svg" ) );
+  mIcon = QgsApplication::getThemeIcon( QStringLiteral( "/mIndicatorNonRemovable.svg" ) );
 
   QgsLayerTree *tree = mLayerTreeView->layerTreeModel()->rootGroup();
   onAddedChildren( tree, 0, tree->children().count() - 1 );