Dark Reader inspired me to write a similar one for myself: https://chrome.google.com/webstore/detail/obscura/nhlkgnilpm...

Where Dark Reader is full-featured, mine is minimal. It adds a translucent tinted div overlay on every page, with mix-blend-mode:multiply to make the text come out a little sharper. No themes or querying page elements.

You can audit the code yourself if you want, it's only 58 lines of JS across 2 files:

    // content.js (this runs on every page):
    'use strict';
    const C=chrome, d=document.createElement('div'),
        update=(o={})=>{
            if('tog' in o){d.style.height = (o.tog.newValue | o.tog)+'00%';}
            if('color' in o){d.style.backgroundColor = o.color.newValue || o.color;}
        };

    d.style='mix-blend-mode:multiply; z-index:2147483647; position:fixed; width:100%; top:0; right:0; pointer-events:none;';
    document.documentElement.insertBefore(d, document.documentElement.firstChild);
    C.storage.local.get(null, update);
    C.runtime.onMessage.addListener(update);
    C.storage.onChanged.addListener(update);


    // popup.js:
    'use strict';
    let XY=[150,40];
    const C=chrome, S=C.storage.local,
        send=x=>C.tabs.query({active:true,currentWindow:true}, t=>C.tabs.sendMessage(t[0].id, x)),
        torgba=x=>`rgba(${x[0]}, ${x[1]}, ${x[2]}, ${x[3]/255})`,
        dropper=(x,y)=>torgba(c.getImageData(x,y,1,1).data),
        target=(x,y)=>{c.fillStyle = 'black'; c.beginPath(); c.arc(x,y,30,0,Math.PI*2,true); c.stroke();},
        touch_xy=t=>{const r=c.canvas.getBoundingClientRect(), f=(x,ox,r)=>Math.floor(Math.min(Math.max(x,ox+1),r-1)-ox);
                    return[f(t.pageX, r.left, r.right), f(t.pageY, r.top, r.bottom)];},
        init_c=()=>{
            c.clearRect(0,0,c.canvas.width,c.canvas.height);
            c.canvas.style.backgroundColor='white';
            const g1=c.createLinearGradient(0, 0, c.canvas.width, 0), // horiz, hue
                    g2=c.createLinearGradient(0, 0, 0, c.canvas.height);  // vert, lightness
            g1.addColorStop(0.2,'rgba(255,120,120,0)');
            g1.addColorStop(1,'rgba(255,130,70,0.99)');
            g2.addColorStop(0,'rgba(0,0,0,0)');
            g2.addColorStop(1,'rgba(0,0,0,0.8)');
            c.fillStyle = g1; c.fillRect(0,0,c.canvas.width,c.canvas.height);
            c.fillStyle = g2; c.fillRect(0,0,c.canvas.width,c.canvas.height);},
        xy_send=(xy)=>{init_c(); target(...xy); send({xy, color:dropper(...xy)});},
        xy_save=(xy)=>{init_c(); target(...xy); S.set({xy, color:dropper(...xy)});},
        c=document.getElementById('c').getContext('2d'),
        tog=document.getElementById('tog');

    window.addEventListener('load',()=>{
        S.get(null, o=>{
            if('tog' in o){tog.checked = o.tog;} else {S.set({tog:tog.checked});}
            if('xy' in o){init_c(); target(...o.xy); XY=o.xy;} else {S.set({xy:XY});} // color's xy coords in canvas
            if(!('color' in o)){S.set({color:dropper(...XY)});} // hardcoded default xy location
        });
        tog.addEventListener('change', x=>{S.set({tog:tog.checked});});
        c.canvas.addEventListener('mousemove', x=>{if(x.buttons!==1){return;} xy_send([x.offsetX, x.offsetY]);});
        c.canvas.addEventListener('mouseup', x=>xy_save([x.offsetX, x.offsetY]));
        c.canvas.addEventListener('touchmove', x=>xy_send(touch_xy(x.targetTouches[0])));
        c.canvas.addEventListener('touchend', x=>xy_save(touch_xy(x.changedTouches[0])));
    });
    window.addEventListener('keydown', e=>{
        if(e.key==='x'){tog.checked = !tog.checked; S.set({tog:tog.checked});}
    });

Full source: https://gitlab.com/hoosieree/chrome-extension-obscura