Make Text element a sibling of the Rectangle to avoid the text from depending on the rectangle's opacity property. Switch to Nheko theme colors

resources/qml/delegates/ImageMessage.qml

@@ -46,25 +46,25 @@ Item {
 
             Rectangle {
                 id: container
+                
                 width: parent.width
                 implicitHeight: imgcaption.implicitHeight
                 anchors.bottom: overlay.bottom 
-                color: "black"
+                color: colors.window
                 opacity: 0.75
+            }
 
             Text {
                 id: imgcaption
                 
-                    anchors.fill: parent
+                anchors.fill: container
                 elide: Text.ElideMiddle
                 horizontalAlignment: Text.AlignHCenter
                 verticalAlignment: Text.AlignVCenter
                 // See this MSC: https://github.com/matrix-org/matrix-doc/pull/2530
                 text: model.data.filename ? model.data.filename : model.data.body
-                    color: "white"
+                color: colors.text
             }
         }
     }
-        
-    }
 }