Edit: I guess I should actually read the post. You need a physical button. Disregard lol

I added the following lines to MagicMirror/css/custom.css

.male {
	background: black;
	color: blue;
}

.female {
	background: white;
	color: pink;
}

and for testing I edited the clock module (MagicMirror/modules/default/clock/clock.js) because it get’s updated every second.

put those lines right after line 69

if(secondsWrapper.innerHTML % 10 == 0){ /*changes class every 10 seconds, because i don't have a button i made this "hack" for testing purposes*/
			var body = document.querySelector('body');
			if(body.classList.contains("male")){
				body.classList.remove("male");
				body.classList.add("female");
			} else {
				body.classList.remove("female");
				body.classList.add("male");
			}
		}

'use strict';

/* Magic Mirror
 * Module: MMM-PIR-Sensor
 *
 * By Paul-Vincent Roll http://paulvincentroll.com
 * MIT Licensed.
 */

/*import dependencies*/
const NodeHelper = require('node_helper');
const gpio = require('wiring-pi');
const exec = require('child_process').exec;

module.exports = NodeHelper.create({
  /*this method got fired when the core is starting your module helper*/
  start: function () {
    this.started = false
  },
  /*this function will activate your monitor by enabling hdmi output or sending a signal to the relais via gpio pin*/
  activateMonitor: function () {
    /* you don't need the following 3 lines, which would set a signal to the relais specified in the config section of your module of the file above*/
    if (this.config.relayPIN != false) {
      gpio.digitalWrite(this.config.relayPIN, this.config.relayOnState)
    }
    /*after removing the previous 3 lines you also need to change "else if" to "if". the following lines run a command in a child process in the background to enable the hdmi output*/
    else if (this.config.relayPIN == false){
      exec("/opt/vc/bin/tvservice -p", null);
    }
  },
/*this function will deactivate your monitor by disabling hdmi output or sending a signal to the relais via gpio pin*/
  deactivateMonitor: function () {
/* you don't need the following 3 lines, which would set a signal to the relais specified in the config section of your module of the file above*/
    if (this.config.relayPIN != false) {
      gpio.digitalWrite(this.config.relayPIN, this.config.relayOffState)
    }
 /*after removing the previous 3 lines you also need to change "else if" to "if". the following lines run a command in a child process in the background to disable the hdmi output*/
    else if (this.config.relayPIN == false){
      exec("/opt/vc/bin/tvservice -o", null);
    }
  },
  /*here you receive messages which you send via modulename.js*/
  socketNotificationReceived: function(notification, payload) {
    const self = this;
    /*the following part runs when you receive the notification from your module l. 33 on github "this.sendSocketNotification('CONFIG', this.config);"*/
    if (notification === 'CONFIG' && this.started == false) {
      
      const self = this
      this.config = payload
      
      //Setup pins
      exec("echo '" + this.config.sensorPIN.toString() + "' > /sys/class/gpio/export", null); /*motion sensor pin*/
      exec("echo 'in' > /sys/class/gpio/gpio" + this.config.sensorPIN.toString() + "/direction", null); /*relais pin*/
      /*fllowing lines not important if you're not using relais*/
      if (this.config.relayPIN) {
        exec("echo '" + this.config.relayPIN.toString() + "' > /sys/class/gpio/export", null);
        exec("echo 'out' > /sys/class/gpio/gpio" + this.config.relayPIN.toString() + "/direction", null);
        exec("echo '1' > /sys/class/gpio/gpio" + this.config.relayPIN.toString() + "/value", null);
      }
      
      //Set gpio-mode
      gpio.setup('sys');
      
      //Detected movement
      /*when you detect a movement with your sensor it will run the callback "function(delta){...}"*/
      gpio.wiringPiISR(this.config.sensorPIN, gpio.INT_EDGE_BOTH, function(delta) {
        if (gpio.digitalRead(self.config.sensorPIN) == 1) {
          /* motion detected send the notification to your module*/
          self.sendSocketNotification("USER_PRESENCE", true);
          if (self.config.powerSaving){
            /*activate the monitor again to show the user data on your screen*/
            self.activateMonitor()
          }
        }
        //No movement
        else if (gpio.digitalRead(self.config.sensorPIN) == 0) {
          /*send your module the notification tha there is no user present and deactivate the monitor*/
          self.sendSocketNotification("USER_PRESENCE", false);
          if (self.config.powerSaving){
            self.deactivateMonitor()
          }
        }
      });
     
    this.started = true
    };
  }
  
});

this references the module itself

so in the module you can do this.sendSocketNotification(…) which will call the function sendsocketnotification of your module (this)

also you can follow this guide to develop a module https://github.com/MichMich/MagicMirror/tree/master/modules

/* Magic Mirror
 * Module: MMM-PIR-Sensor
 *
 * By Paul-Vincent Roll http://paulvincentroll.com
 * MIT Licensed.
 */

/* registers a module with the name MMM-PIR-Sensor in the magic mirror system*/
Module.register('MMM-PIR-Sensor',{
	
        /*config part*/
	defaults: {
		sensorPIN: 22, /*number of pin can be different for raspberry pi version e.g. https://raspilab.files.wordpress.com/2014/08/gpiosb.png*/
		relayPIN: false, /*this is for using relais in this module not needed because you can also set the hdmi output off*/
		powerSaving: true,
		relayOnState: 1, /*not important because you're not using relays*/
	},
	
        /*here you receive messages which you send via node_helper.js*/
	socketNotificationReceived: function(notification, payload) {
		if (notification === "USER_PRESENCE"){
                        /*when you receive the notification user_presence from your node_helper you send a global message with this notification to every module in magic mirror so they can handle this event as well*/
			this.sendNotification(notification, payload)
		}
	},
	
        /*this method got fired when the core is starting your module
	start: function() {
                /* the following 6 lines can be removed if you don't want to use relais. The functionality of these lines is to set relayoffstate depending on the relayonstate defined in the configt section above*/
		if (this.config.relayOnState == 1){
			this.config.relayOffState = 0
		}
		else if (this.config.relayOnState == 0){
			this.config.relayOffState = 1
		}
                /*here you send a notification to your node_helper.js*/
		this.sendSocketNotification('CONFIG', this.config);
                 /*here you print some information to the console, which you can't se without enabling the developer console in the electronh wrapper during runtime*/
		Log.info('Starting module: ' + this.name);
	}
});

then you should write your own module where you can control your pins e.g. like this module https://github.com/paviro/MMM-PIR-Sensor

instead of shutting down your tv you could change classes of the document to change the design