1.开发环境：Android studio 3.4.0，kotlin实现
2.开发准备
①在values中添加资源文件【“new“ → "Values resouce file”】定义为”custom_theme_attrs“，内容如下：

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- 控制app背景色 format:颜色值、资源引用 -->
    <attr name="custom_attr_app_bg" format="color|reference" />
    <!-- 控制app标题栏背景色 format:颜色值、资源引用 -->
    <attr name="custom_attr_app_title_layout_bg" format="color|reference" />
    <!-- 用户头像显示占位Drawable format:颜色值、资源引用 -->
    <attr name="custom_attr_user_photo_place_holder" format="color|reference" />
    <!-- 用户昵称字体颜色 format:颜色值、资源引用 -->
    <attr name="custom_attr_nickname_text_color" format="color|reference" />
    <!-- 用户备注字体颜色 format:颜色值、资源引用 -->
    <attr name="custom_attr_remark_text_color" format="color|reference" />
    <!-- 用户头像显示的透明度 format:尺寸值、资源引用 -->
    <attr name="custom_attr_user_photo_alpha" format="dimension|reference" />

    <attr name="titlebar_bg" format="color|reference"/>
    <attr name="titlebar_text_color" format="color|reference"/>
    <attr name="body_bg" format="color|reference"/>
    <attr name="body_text_color" format="color|reference"/>
    <attr name="title" format="string"/>
</resources>

②在资源文件”styles“中添加如下代码【添加主题】

<!-- 主题：白天 -->
    <style name="theme_day" parent="Theme.AppCompat.Light.NoActionBar">
        <item name="titlebar_bg">#1EABF0</item>
        <item name="titlebar_text_color">#FFFFFF</item>
        <item name="body_bg">#F3F3F3</item>
        <item name="body_text_color">#333333</item>
    </style>
    
    <!-- 主题：夜晚 -->
    <style name="theme_night" parent="Theme.AppCompat.Light.NoActionBar">
        <item name="titlebar_bg">#000000</item>
        <item name="titlebar_text_color">#FFFFFF</item>
        <item name="body_bg">#888888</item>
        <item name="body_text_color">#EAEAEA</item>
    </style>

    <!-- theme_day.chi_sim 表示继承theme_day -->
    <style name="theme_day.chi_sim">
        <item name="title">主题配置</item>
    </style>
    <style name="theme_night.eng">
        <item name="title">TopicConfig</item>
    </style>

3.添加观察者接口：

package com.zjhj.maxapp.theme

interface ThemeChangeObserver {
    /**
     * 加载当前主题
     */
    fun loadingCurrentTheme()

    /**
     * 主题改变后通知
     */
    fun notifyThemeChanged()
}

4.在BaseActivity中实现接口

package com.zjhj.maxapp.base

import android.content.Context
import android.os.Bundle
import android.view.WindowManager
import androidx.appcompat.app.AppCompatActivity
import com.zjhj.maxapp.App
import com.zjhj.maxapp.R
import com.zjhj.maxapp.theme.ThemeChangeObserver
import com.zjhj.maxapp.utils.L //自定义日志类


/**
 * CreateTime 2020/4/2 09:10
 * Author LiuShiHua
 * Description：
 */
abstract class BaseActivity : AppCompatActivity(), ThemeChangeObserver {

    private val KEY_THEME_TAG = "myThemeTag"
    private var isChangeTheme: Boolean = false
    override fun onCreate(savedInstanceState: Bundle?) {
        window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)
        window.addFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN)
        setThemeBeforeCreate()
        super.onCreate(savedInstanceState)
        setContentView()
        initView()
        initData()
        getData()
    }

    /**
     */
    private fun setThemeBeforeCreate() {
        App.registerObserver(this)//将当前Acitivity注册成观察者
        loadingCurrentTheme()
    }

    /**
     * 加载当前主题
     * 需要在super.onCreate(savedInstanceState)之前执行才会生效
     */
    override fun loadingCurrentTheme() {
        when (getThemeTag()) {
            1 -> setTheme(R.style.theme_day_chi_sim)
            -1 -> setTheme(R.style.theme_night_eng)
        }
        L.d("loadingCurrentTheme:" + this::class.java)
    }

    abstract fun setContentView()
    abstract fun initView()
    abstract fun initData()
    abstract fun getData()

    /**
     * 获取当前主题类型
     */
    protected open fun getThemeTag(): Int {
        val preferences = getSharedPreferences("MaxTheme", Context.MODE_PRIVATE)
        return preferences.getInt(KEY_THEME_TAG, 1)
    }

    /**
     * 设置主题类型
     * 保存在sharedprferences中
     */
    protected open fun setThemeTag(tag: Int) {
        L.d("setThemeTag:" + tag)
        val preferences = getSharedPreferences("MaxTheme", Context.MODE_PRIVATE)
        val edit = preferences.edit()
        edit.putInt(KEY_THEME_TAG, tag)
        edit.commit()
        App.notifyByThemeChanged()
    }

    /**
     * 注销这个观察者
     */
    override fun onDestroy() {
        App.unregisterObserver(this)
        super.onDestroy()
    }
}

5.在APP（继承 Application）中管理观察者

package com.zjhj.maxapp

import android.app.Application
import android.content.Context
import com.zjhj.maxapp.theme.ThemeChangeObserver


/**
 * CreateTime 2020/4/9 09:40
 * Author LiuShiHua
 * Description：
 */
class App : Application() {


    companion object {//静态属性或方法
        lateinit var context: Context
        get

        private var mThemeChangeObserverStack: MutableList<ThemeChangeObserver>? = null

        /**
         * 获得observer堆栈
         */
        private fun obtainThemeChangeObserverStack(): MutableList<ThemeChangeObserver>? {
            if (mThemeChangeObserverStack == null) mThemeChangeObserverStack = ArrayList()
            return mThemeChangeObserverStack
        }

        /**
         * 向堆栈中添加observer
         */
        fun registerObserver(observer: ThemeChangeObserver?) {
            if (observer == null || obtainThemeChangeObserverStack()!!.contains(observer)) return
            obtainThemeChangeObserverStack()!!.add(observer)
        }

        /**
         * 从堆栈中移除observer
         */
        fun unregisterObserver(observer: ThemeChangeObserver?) {
            if (observer == null || !obtainThemeChangeObserverStack()!!.contains(observer)) return
            obtainThemeChangeObserverStack()!!.remove(observer)
        }

        /**
         * 向堆栈中所有对象发送更新UI的指令
         */
        fun notifyByThemeChanged() {
            val observers: List<ThemeChangeObserver>? = obtainThemeChangeObserverStack()
            for (observer in observers!!) {
                observer.loadingCurrentTheme() //
                observer.notifyThemeChanged() //
            }
        }
    }

    override fun onCreate() {
        super.onCreate()
        context = this
    }
}

5.在BaseActivity实例中去调用更换主题

package com.zjhj.maxapp

import com.zjhj.maxapp.base.BaseActivity
import kotlinx.android.synthetic.main.activity_theme.*

class ThemeActivity : BaseActivity() {
    override fun setContentView() {
        setContentView(R.layout.activity_theme)
    }

    override fun initView() {
        changeTheme.setOnClickListener {
            if (getThemeTag()==1) {//设置主题
                setThemeTag(-1)
            } else {
                setThemeTag(1)
            }
            recreate()//重新构建页面
        }
    }

    override fun initData() {

    }

    override fun getData() {

    }

    override fun notifyThemeChanged() {

    }
}

6.说明：网上有些建议是使用recreate()来重新构建页面就行，实际中很多Activity都无法使用recreate()来重新构建页面，或者想保存之前的页面信息而不希望页面重新构建，我这里使用的是在Activity中notifyThemeChanged中重新设置对应主题的属性值【感觉比较low】

...
 override fun notifyThemeChanged() {
        val typedValue = TypedValue()
        //获取当前主题下的titlebar_bg属性值，并赋给typedValue
        theme.resolveAttribute(R.attr.titlebar_bg, typedValue, true)
        //设置属性值【注意：属性是那种类型，设置的时候就用成那种类型】
        toolBar.setBackgroundColor(typedValue.data)
    }
...

实现效果：在主题设置也改变主题后其他页面跟这变化