use the primary color for the brush size seekbar

app/src/main/kotlin/com/simplemobiletools/draw/activities/MainActivity.kt

@@ -79,6 +79,7 @@ class MainActivity : SimpleActivity(), MyCanvas.PathsChangedListener {
         val isStrokeWidthBarEnabled = config.showBrushSize
         stroke_width_bar.beVisibleIf(isStrokeWidthBarEnabled)
         my_canvas.setIsStrokeWidthBarEnabled(isStrokeWidthBarEnabled)
+        updateTextColors(main_holder)
     }
 
     override fun onPause() {

app/src/main/res/layout/activity_main.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <RelativeLayout
     xmlns:android="http://schemas.android.com/apk/res/android"
+    android:id="@+id/main_holder"
     android:layout_width="match_parent"
     android:layout_height="match_parent">
 
@@ -26,7 +27,7 @@
         android:src="@drawable/ic_undo"
         android:visibility="gone"/>
 
-    <SeekBar
+    <com.simplemobiletools.commons.views.MySeekBar
         android:id="@+id/stroke_width_bar"
         android:layout_width="@dimen/stroke_bar_size"
         android:layout_height="wrap_content"