style.xml

<style name="Theme.AppCompat.Translucent" parent="Theme.AppCompat.NoActionBar">
        <item name="android:background">#33000000</item>
        <item name="android:windowNoTitle">true</item>
        <item name="android:windowBackground">@android:color/transparent</item>
        <item name="android:colorBackgroundCacheHint">@null</item>
        <item name="android:windowIsTranslucent">true</item>
        <item name="android:windowAnimationStyle">@android:style/Animation</item>
    </style>

TransparentActivity.kt

class TransparentActivity : AppCompatActivity() {
    companion object {
        fun createIntent(context: Context): Intent = Intent(context, TransparentActivity::class.java)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_transparent)

        val rootView = window.decorView.findViewById<View>(android.R.id.content).rootView
        rootView.viewTreeObserver.let { observer ->
            observer.addOnGlobalLayoutListener {
                Completable.create {
                    val file = File(Environment.getExternalStoragePublicDirectory(
                            Environment.DIRECTORY_PICTURES), "Screenshots")

                    val date = SimpleDateFormat("yyyyMMdd_HHmmss").format(Date())

                    try {
                        rootView.isDrawingCacheEnabled = true
                        val bitmap = Bitmap.createBitmap(rootView.drawingCache)
                        rootView.isDrawingCacheEnabled = false

                        val imgFile = File(file.path + File.separator +
                                "IMG_" + date + ".jpg")

                        val outputStream = FileOutputStream(imgFile)
                        bitmap.compress(Bitmap.CompressFormat.JPEG, 100, outputStream)
                        outputStream.flush()
                        outputStream.close()
                        it.onComplete()
                    } catch (e: Throwable) {
                        it.onError(e)
                        e.printStackTrace()
                    }

                }
                        .subscribeOn(AndroidSchedulers.mainThread())
                        .observeOn(AndroidSchedulers.mainThread())
                        .subscribe({
                            Toast.makeText(applicationContext, "take photo", Toast.LENGTH_SHORT).show()
                        }, {
                            Log.d("log", it.message)
                            Toast.makeText(applicationContext, "fail take photo\n ${it.message}", Toast.LENGTH_SHORT).show()
                        })
            }
        }


    }

}