FIX: Icon fix for action maps buttons in background and resolution (#2314)

Author: josepmariapujol-unity

Packages/com.unity.inputsystem/InputSystem/Editor/UITKAssetEditor/PackageResources/InputActionsEditor.uxml

@@ -21,7 +21,7 @@
                 <ui:VisualElement name="action-maps-container" class="body-panel-container actions-container">
                     <ui:VisualElement name="header" class="body-panel-header">
                         <ui:Label text="Action Maps" display-tooltip-when-elided="true" style="flex-grow: 1;" />
-                        <ui:Button display-tooltip-when-elided="true" name="add-new-action-map-button" style="align-items: auto; background-color: rgba(255, 255, 255, 0);" />
+                        <ui:Button display-tooltip-when-elided="true" name="add-new-action-map-button" style="align-items: auto;" />
                     </ui:VisualElement>
                     <ui:VisualElement name="body">
                         <ui:ListView focusable="true" name="action-maps-list-view" />
@@ -32,7 +32,7 @@
                     <ui:VisualElement name="actions-container" class="body-panel-container">
                         <ui:VisualElement name="header" class="body-panel-header" style="justify-content: space-between;">
                             <ui:Label text="Actions" display-tooltip-when-elided="true" name="actions-label" />
-                            <ui:Button display-tooltip-when-elided="true" name="add-new-action-button" style="align-items: auto; background-color: rgba(188, 188, 188, 0);" />
+                            <ui:Button display-tooltip-when-elided="true" name="add-new-action-button" style="align-items: auto;" />
                         </ui:VisualElement>
                         <ui:VisualElement name="body">
                             <ui:TreeView view-data-key="unity-tree-view" focusable="true" name="actions-tree-view" show-border="false" reorderable="true" show-alternating-row-backgrounds="None" fixed-item-height="20" />

Packages/com.unity.inputsystem/InputSystem/Editor/UITKAssetEditor/PackageResources/InputActionsEditorStyles.uss

@@ -175,30 +175,34 @@
 }
 
 .add-binging-button-dark-theme {
-    width: 14px;
-    height: 14px;
+    width: 16px;
+    height: 16px;
     background-image: resource('d_Toolbar Plus More.png');
+    background-color: transparent;
     -unity-background-scale-mode: scale-to-fit;
 }
 
 .add-binging-button {
-    width: 14px;
-    height: 14px;
+    width: 16px;
+    height: 16px;
     background-image: resource('Toolbar Plus More.png');
+    background-color: transparent;
     -unity-background-scale-mode: scale-to-fit;
 }
 
 .add-button-dark-theme {
-    width: 18px;
-    height: 18px;
+    width: 16px;
+    height: 16px;
     background-image: resource('d_Toolbar Plus.png');
+    background-color: transparent;
     -unity-background-scale-mode: scale-to-fit;
 }
 
 .add-button {
-    width: 18px;
-    height: 18px;
+    width: 16px;
+    height: 16px;
     background-image: resource('Toolbar Plus.png');
+    background-color: transparent;
     -unity-background-scale-mode: scale-to-fit;
 }