The worst part here was to find the “name” for this item and how it is called inside the theme. So I looked through every drawable in the android SDK folder and finally found the drawables named “text_select_handle_middle”, “text_select_handle_left” and “text_select_handle_right”.
So the solution is simple: Add these drawables with customized design/color to your drawable folder and add them to your theme style definition like:
<style name="MyCustomTheme" parent="@style/MyNotSoCustomTheme">
<item name="android:textSelectHandle">@drawable/text_select_handle_middle</item>
<item name="android:textSelectHandleLeft">@drawable/text_select_handle_left</item>
<item name="android:textSelectHandleRight">@drawable/text_select_handle_right</item>
</style>