This I know and it's already a huge improvement over the previous versions.
That version which come with update4 (or even update5 as far as i know), still very limited by the ability to change everything. Its only add some bits (like manuall changing of the images, in the same old - boring way, some bits here and here), but new mui which will come in os4.2, will bring a lot more. Not like "a bit new", but totally and fully new concept of mui preferences, where you can change just absolutly everything about which you can think. Any kind of buttons, any kind of gadgets, any kind of text. You can choice any kind of color, gradient, images for background for any of those objects (i.e. you even can put backgorund for text everythere, or you can choice the looks of buttons to any you want).
In other words, with new mui you will be able to mimik any look : kde, winxp, morphos-ferox one, haiku, gnome, just absolutly everything. Currently, that version which we have in betatest are 1:1 the same as mui4 on morphos if you in interest to know. Only very-very small bits missing, which i sure Thore will add for final release with os4.2
it it isn't very smooth to try and make everything look the same as ReAction as soon as you change the theme. I think that there are room for both options, one that copies the look from reaction, or parts of it, and the options to change the look directly in MUI that are available now. Something that works like the option to use the "new look" lister but for other components like buttons.
Our current public mui have "presets" , which exactly mean "skin" for mui. As public mui still lack a lof ot stuff for making good looking design, users still forced for now to use current and old inbuild os4-default-mui-theme. Which, are done with having in mind all the limitations of the old mui (that is why it looks outdated). But even now, you can make some new preset, which will use new images and some new stuff, which, you will just choice from mui via "open preset". And yes, its all still not what you want because public version still not as it now in beta.
With new version, you will even have "setting" button at top, where you just can choie any preset you want (i.e. skin). If Mason will do new theme for new mui, then it can be inbuild by default, and that one can be 1:1 as default aos4 look. And ppls when will do new themes for aos4, will be now abble to do it not only for workbench/reaction, but also and for mui (i.e. like morphos designers do: they all the time release a themes where is and ambient and mui themes included).
So, just wait for 4.2, that will brings a lot in terms of mui.