It's not a dark mode problem.
I do see a problem on initial launch though, when using private mode, but switching the colors off and on fixed it. I'm not sure why you're not seeing the colors, but it does look like this part of the game can be improved.
The colors actually work with Safari when I switch the Colors option on/off/on.
Unfortunately, this does not work in Firefox and Chrome.
However, I also noticed in Chrome that changes to the options are NOT permanently saved there.
The next time I open the options, everything is back to default.
And—if you do decide to revisit the project—it would be nice if you could click directly on the round “handle” of each option to change a value.
Currently, you always have to click on the black line behind of the switch to move it.
I could imagine that not every user would understand this immediately.
And oh yes, none of the three browsers are running in private mode. And in FF and Chrome, all ad blockers were disabled for this test.
I did minor update to Circles With Grandma today. The color setting should work now.
new SpeechSynthesisUtterance() is not working though on Safari though. I think it might be related to the “breaking the sound barrier” issue, where web browsers won't let you play any sounds without user interaction.
Yet, it works on a Chromium browser. 
Hype has a function hypeDocument.functions() which I used to load the custom “resetSpeech() function. The parameter arguments hypeDocument, element, event were not passed through by default. So, I added it as an argument. That's when event showed some data, but it didn't solve the problem.
At least the game is playable now. It was totally broken before.
It's a slider, with a single step. So, that is confusing. It probably should be a checkbox, since it's a boolean value. There's even a new “switch” option.
— https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/input/checkbox
But right now, I'm going to eat breakfast. 
UPDATE: I think I tried the checkbox before. Because when I tried changing the input type just now, I didn't like it. It's blurry when scaled. I tried it with the switch, which looks nice, but it's also blurry when scaled.