I have thought about what you encountered and I see a few possible variations:
1 use the coloured icons.
2 use white icons when the theme is dark
It is okay for the app, but on OSX you won't see the icons in the menu if theme is light. For OSX dark it is okay.
3 invert the white icons when the theme is white ( they will be black )
Now it is okay in OSX light, but in the App you won't see them. Inverting means having more than one icon set in the app? Complexity will rise a lot for such a feature. And I did not search if I can read which theme is loaded in OSX.
4 use a grey color that works in both dark and light theme.
In principle yes, but OSX light is a light grey - so then we have also a bad contrast between icons and background

On Windows it will always look good with white icons, because the menu is inside the app and always black. But we always must think on all OS with all themes... and sometimes even this is not enough

I'm happy to contribute to making this work. and at the same time understand it not the most important thing. and that to save complicating te project only one theme is used.
It just for me personally I prefer a clean icon. it can even be a coloured one. so the good thing is that the project is open source and I was able to change the icons the way I want them and make a new build.
and like to share back to you what I did.
Yes, thanks for sharing! So don't understand my wrong. Most of the features in MLVApp are good ideas from users - that is very important for us!
And yes - it is open source. So, if you don't like the design, you can change what you want for your personal version. If you like to play with that - all what is called darkstyle in the code has many parameters, which make the app look as it is. You can change all colours like you like them!

darkstyle.h
darkstyleOSX.qss (on OSX)
darkstyle.qss (non OSX)
What I started one day (but I stopped because it was too much work for my taste) was another darkstyle with orange instead of blue... a bit Resolve like
