10/14 update
Went smoothly for Gnome 43 except lost the app grid button from the cosmic dock.... Maybe it fell on the floor, I'll have a look.. 
Also having issue with nautilus, can't resize left panel and F9 shortcut to show or hide it doesn't work.
Right clic menu also looks weird, need to try several time to have a clean one.
Also having the resize panel issue and now Nautilus is not showing in dark modus (black as any other window) but in gray, not matching any other window... Does anybody have the same issue?
The update process went without any problem...
Jupp same problem!! Nautilus is pure white..doesnt follow themes any more..
- Edited
Some preliminary workarounds/explanations here:
Appearance of Nautilus and other apps: The GNOME project keeps moving more and more apps to libadwaita theming, the latest being, among others, the file manager Nautilus. This means that it no longer respects the system GTK widget theme (as set in e.g. Budgie Desktop Settings), and also might not adjust to dark themes correctly.
For the second problem (not adjusting to dark system themes) there are two possible solutions. One is to open gnome-control-center (from a terminal in Budgie as it's not shown in the menu anymore), navigate to the "Appearance" section and switch between "Default" and "Dark" there. Note that libadwaita has some integrated dark theme detection, which will automatically switch to dark mode, if your widget theme ends in "-Dark". I'd have to research how to override that if that's not what you want.
The other possible solution using dconf-editor is described here: https://discuss.getsol.us/d/8463-fix-for-dark-themes-with-blaring-white-applications
For the first problem (not respecting system theme) one solution/workaround is to patch the libadwaita theme to match your system widget theme colour. An easy way to do this is by using the Gradience app. Currently available as a flatpak here but we'll probably see about integrating it into the Solus repository soon. With this app you can either manually adjust the theme colours (if you have the time and inclination to do that) or, as I did, explore pre-configured presets. Personally I use our ant-dracula-gtk-theme, so I searched for the Dracula preset and applied it. This will adjust the libadwaita theme colours to match the widget theme. Not perfect, but much more agreeable to the eyes.
So much for the previously known issues. Some of the stuff mentioned in this thread was new to me. I'll have to do some research on those issues.
Sorry for any possible errors in this post. Written in somewhat of a rush before I have to leave my apartment.
Seems like a good workaround for now..
rjuarezp now Nautilus is not showing in dark modus
I noticed that there was no such problem with Caja when I updted my MATE VM, so I tried installing it in Budgie, and it's good there also. Dolphin is also unaffected by this issue, but adding Dolphin to Budgie would probably add a lot of dependencies. Adding Caja adds just two packages. So, if you're really annoyed looking that that white Nautilus window, you might want to give Caja a try for now.
WetGeek Thanks a lot! Appreciate the help! I will think about this or the workaround from @Staudey... or maybe leaving it as it is until a final solution is found.
Appreciate a lot the work being done with Solus. Awesome Linux distribution. Very happy to have this distro (I am linux user since over 25 years now!!!).
is it true you cant resize the left pane in nautilus, and that the expandable folders are gone?
Staudey Appearance of Nautilus and other apps: The GNOME project keeps moving more and more apps to libadwaita theming, the latest being, among others, the file manager Nautilus. This means that it no longer respects the system GTK widget theme (as set in e.g. Budgie Desktop Settings), and also might not adjust to dark themes correctly.
since there was no workaround for my problem, I'm going to assume the tiny font is the new normal?
Lucien_Lachance Arf, yes it's true :/