attrs.xml

<resources>
    <declare-styleable name="ThemeableMediaRouteButton">
        <!-- This drawable is a state list where the "checked" state
             indicates active media routing.  Checkable indicates connecting
             and non-checked / non-checkable indicates
             that media is playing to the local device only. -->
        <attr name="routeEnabledDrawable" format="reference" />
        <attr name="iconColor" format="reference|color" />
        <attr name="android:minWidth" />
        <attr name="android:minHeight" />
    </declare-styleable>
</resources>

menu_cast.xml

 <menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto" >
 
    <item
        android:id="@+id/menu_cast_item"
        android:icon="@drawable/ic_chrome_off"
        android:orderInCategory="0"
        app:showAsAction="always"
        app:actionProviderClass="com.ryanharter.mediaroute.widgets.ThemeableMediaRouteActionProvider"
        android:title="@string/menu_cast"/>
 </menu>

styles.xml

<resources>
    <style name="Widget.MediaRouter.MediaRouteButton" parent="Widget.MediaRouter.Light.MediaRouteButton">
        <item name="routeEnabledDrawable">@drawable/ic_chrome_media_route</item>
        <item name="iconColor">@color/red</item>
    </style>

    <style name="Widget.MediaRouter.MediaRouteButton.Section1">
        <item name="iconColor">@color/white</item>
    </style>

    <style name="Widget.MediaRouter.MediaRouteButton.Section2">
        <item name="iconColor">@color/blue</item>
    </style>
</resources>

ThemeableMediaRouteActionProvider.java

package com.ryanharter.mediaroute.widgets;

import android.content.Context;
import android.support.v7.app.MediaRouteActionProvider;
import android.support.v7.app.MediaRouteButton;

/**
 * A MediaRouteActionProvider that allows the use of a ThemeableMediaRouteButton.
 */
public class ThemeableMediaRouteActionProvider extends MediaRouteActionProvider {

    public ThemeableMediaRouteActionProvider(Context context) {
        super(context);
    }

    @Override public MediaRouteButton onCreateMediaRouteButton() {
        return new ThemeableMediaRouteButton(getContext());
    }
}

ThemeableMediaRouteButton.java

package com.ryanharter.mediaroute.widgets;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.PorterDuff;
import android.graphics.drawable.Drawable;
import android.support.v4.graphics.drawable.DrawableCompat;
import android.support.v7.app.MediaRouteButton;
import android.util.AttributeSet;

import com.ryanharter.mediaroute.R;

/**
 * Allows theming of the MediaRouteButton using the theme associated with 
 * the context passed in.
 */
public class ThemeableMediaRouteButton extends MediaRouteButton {

    private static final String TAG = ThemeableMediaRouteButton.class.getSimpleName();

    private int mMinWidth;
    private int mMinHeight;
    private int mColor;
    private Drawable mRemoteIndicator;

    public ThemeableMediaRouteButton(Context context) {
        this(context, null);
    }

    public ThemeableMediaRouteButton(Context context, AttributeSet attrs) {
        this(context, attrs, R.attr.mediaRouteButtonStyle);
    }

    public ThemeableMediaRouteButton(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.ThemeableMediaRouteButton, defStyleAttr, 0);
        mColor = a.getColor(R.styleable.ThemeableMediaRouteButton_iconColor, 0);
        setRemoteIndicatorDrawable(a.getDrawable(
                R.styleable.ThemeableMediaRouteButton_routeEnabledDrawable));
        mMinWidth = a.getDimensionPixelSize(
                R.styleable.ThemeableMediaRouteButton_android_minWidth, 0);
        mMinHeight = a.getDimensionPixelSize(
                R.styleable.ThemeableMediaRouteButton_android_minHeight, 0);

        a.recycle();
    }

    @Override protected void drawableStateChanged() {
        super.drawableStateChanged();

        if (mRemoteIndicator != null) {
            int[] myDrawableState = getDrawableState();
            mRemoteIndicator.setState(myDrawableState);
            invalidate();
        }
    }

    private void setRemoteIndicatorDrawable(Drawable d) {
        if (mRemoteIndicator != null) {
            mRemoteIndicator.setCallback(null);
            unscheduleDrawable(mRemoteIndicator);
        }
        mRemoteIndicator = d;
        if (d != null) {
            d.setColorFilter(mColor, PorterDuff.Mode.SRC_ATOP);
            d.setCallback(this);
            d.setState(getDrawableState());
            d.setVisible(getVisibility() == VISIBLE, false);
        }

        refreshDrawableState();
    }

    @Override protected boolean verifyDrawable(Drawable who) {
        return super.verifyDrawable(who) || who == mRemoteIndicator;
    }

    @Override public void jumpDrawablesToCurrentState() {
        // We can't call super to handle the background so we do it ourselves.
        //super.jumpDrawablesToCurrentState();
        if (getBackground() != null) {
            DrawableCompat.jumpToCurrentState(getBackground());
        }
        // Handle our own remote indicator.
        if (mRemoteIndicator != null) {
            DrawableCompat.jumpToCurrentState(mRemoteIndicator);
        }
    }

    @Override public void setVisibility(int visibility) {
        super.setVisibility(visibility);

        if (mRemoteIndicator != null) {
            mRemoteIndicator.setVisible(getVisibility() == VISIBLE, false);
        }
    }

    @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        final int minWidth = Math.max(mMinWidth,
                mRemoteIndicator != null ? mRemoteIndicator.getIntrinsicWidth() : 0);
        final int minHeight = Math.max(mMinHeight,
                mRemoteIndicator != null ? mRemoteIndicator.getIntrinsicHeight() : 0);
        int width;
        switch (widthMode) {
            case MeasureSpec.EXACTLY:
                width = widthSize;
                break;
            case MeasureSpec.AT_MOST:
                width = Math.min(widthSize, minWidth + getPaddingLeft() + getPaddingRight());
                break;
            default:
            case MeasureSpec.UNSPECIFIED:
                width = minWidth + getPaddingLeft() + getPaddingRight();
                break;
        }
        int height;
        switch (heightMode) {
            case MeasureSpec.EXACTLY:
                height = heightSize;
                break;
            case MeasureSpec.AT_MOST:
                height = Math.min(heightSize, minHeight + getPaddingTop() + getPaddingBottom());
                break;
            default:
            case MeasureSpec.UNSPECIFIED:
                height = minHeight + getPaddingTop() + getPaddingBottom();
                break;
        }
        setMeasuredDimension(width, height);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        if (mRemoteIndicator != null) {
            final int left = getPaddingLeft();
            final int right = getWidth() - getPaddingRight();
            final int top = getPaddingTop();
            final int bottom = getHeight() - getPaddingBottom();

            final int drawWidth = mRemoteIndicator.getIntrinsicWidth();
            final int drawHeight = mRemoteIndicator.getIntrinsicHeight();
            final int drawLeft = left + (right - left - drawWidth) / 2;
            final int drawTop = top + (bottom - top - drawHeight) / 2;

            mRemoteIndicator.setBounds(drawLeft, drawTop,
                    drawLeft + drawWidth, drawTop + drawHeight);
            mRemoteIndicator.draw(canvas);
        }
    }
}

themes.xml

<resources>
    <style name="Theme.MyApp">
        <item name="mediaRouteButtonStyle">@style/Widget.MediaRouter.MediaRouteButton</item>
    </style>
 
    <style name="Theme.MyApp.Section1">
        <item name="mediaRouteButtonStyle">@style/Widget.MediaRouter.MediaRouteButton.Section1</item>
    </style>
 
    <style name="Theme.MyApp.Section2">
        <item name="mediaRouteButtonStyle">@style/Widget.MediaRouter.MediaRouteButton.Section2</item>
    </style>
</resources>

Hi Harter, do you mind publishing a sample? Thanks.

I have just implemented the same requirement, it seems that the cast lib has evolved in the right direction since your gist, setRemoteIndicatorDrawable() is now a public method.
So all that is left is just to call it in the overrided constructor, no need to recopy half the class anymore :)

I've tried step by step... it does not work

I have done it. you are awesome man ^-^