So, this question is kind of an extension of this question, I solved the issue with the headerBackground but I realised that I had manually set the theme of my DatePicker in my layout using android:theme="@style/State0DatePickerTheme. But when I removed that line from my XML, the day selector just turned back to white (my accent colour defined in my base theme). Any thoughts on why this is, or how to fix it? 
For my purposes, I need to have the theme for DatePicker set in my styles.xml with android:datePickerStyle because I'm setting my theme dynamically in my onCreate() and I can't change the theme of the datePicker in java (at least not that I'm aware of).