Any theme you want (For XFCE). The Zorin OS themes are already installed for xfce. This way you can still have the look you enjoyed on Zorin Gnome.
It will default to your last position, so if you want to log into Gnome, you will need to click the gear icon and choose it.
Just a last thing: When I try to click on zorin's appearance from the zorin button it doesn't work... is that normal?
Yes- Zorin Appearance app is for Gnome Only.
From the app menu, (I actually use Whiskermenu on XFCE... on one monitor. The other monitor has zorinmenu lite) categories side, scroll down to SETTINGS, then in the application side > "Appearance." The appearance app in xfce covers Theme, Icon theme and Fonts with DPI scaling.
Once you have selected your theme, back to the menu, Settings - only this time scroll all the way to the bottom for "Window Manager." Open the Window Manager to choose your Window Borders (Something Gnome lacks).
Haha I expected that much...
I will try what you said, thanks again!
Oh btw just tries to reboot my laptop and default is xfce desktop! 
Hmm...
There's still some freezes and other crashes (even worse actually, just moving the mouse around is sloooow and there is a huge delay, I thought it was normal at first because of the classic theme...facepalm) but this time I think I know where it comes from.
I tried to play a 720p h264 video while monitoring my system... and cpu was at 100% and my laptop really hot.
I already got this problem on windows, and I just downloaded the codecs and the gpu driver and it worked fine afterwards.
So knowing that is there something I can do? Maybe change the graphic drivers?
What about Fans and Cooling?
Please install psensor
sudo apt install psensor
and check your computer temps
The cpu cores are around 70°C each, temp1 is 70/80 cpu usage is 3% and memory 40%. Ssd is 50°C.
When I play a video only the cpu usages goes from 3 to 100%. Everything else is the same.
That is HIGH. You really would prefer to never go over 60 degrees c and better to avoid going over 45degress except for very short bursts - I run at about 30 degrees c max.
I think your problem may be a lack of proper cooling. That would definitely cause lag, slowness and graphics trouble.
What is your terminal output of
thermald --version
I thought so... actually my laptop is making a lot of fan noises, but when I took it to an expert he said it was nothing important, eventually the thermal paste to change but he said it didn't recommend it because it would not affect anything...
Also it was fast with w10 and zorin 15 lite.
For your question: It's 1.9.1
WOW!
I make it a point to avoid ever talking bad about another persons mechanical suggestions or opinions.
But...
(cough)
Thermal paste is Crucial in cooling.
If the Fan Management is what is at play, Windows and Z15 may have been utilizing Better Fan Management with faster fan speeds. It is possible that the fans are going out- and the symptoms are now becoming apparent.
It is also possible that the fans are not being properly managed.
So, suggest checking Software Side- first.
Does
sudo apt install lm-sensors
Show it as installed? If not- install it and
sudo sensors-detect
follow All The Prompts to configure.
Run
sensors
Check fan speed:
sensors | grep -i fan
after this... I forget. Im gonna have to look it up,
hang on..
Sorry I hope it's readable...
Optimistic machine... high 100? Wanna sell it? 
Also this can help a lot, too:
sudo apt-get update && sudo apt-get install tlp tlp-rdw
Yeah I smirked at the high 100 too 
Trying the commands brb.
EDIT: The command was successful. Not sure what it did but apparently now when playing a video CPU usage is still low. (Not 100% sure tho because it froze after a moment again )
EDIT 2: After an other hard reboot it seems the cpu usage is still rather high, even if slightly less... (wow and for just a second the temp went up to 98°C!!!)
EDIT 3: After some time the fans started to calm down and now cpu is around 60°C
So apparently normal temperature is lower now (~65°C) but if I play a video or open my browser the cpu usage goes to 100% and temp 80°C.
I think this may bring us back to here...
Do you get any output of
nvidia-settings
are you able to disable Nvidia and test?
Nothing, terminal says not found but can be installed. Should I?
sudo apt install nvidia-settings
Done
What's next?
I have no idea. 
I have never actually used Nvidia Card on a Linux distro. The only thing I do know is that in the Nvidia Settings, somehow, you can turn the Nvidia card off - or set it to not come on...
Would like to test if the Intel dedicated graphics shows the same symptoms with no Nvidia interference.
The only other way to be sure would be to
sudo apt remove --purge nvidia*
Then reinstall with:
sudo ubuntu-drivers autoinstall
edit: I think
prime-select query
Used to show which card you are using - not sure if it still does or requires nvidia prime or ---
Then select card with
sudo prime-select intel
or
sudo prime-select nvidia
Done, at the end it says: Info: the nvidia profile is already set.
After a logout everything seems a bit smoother and the video only takes 80% of the cpu now, so does brave.