Hi all !
Welcome to the second day of a new month. 
Disclaimer: This is my experience with it. Yours could be different.
Today we're going from installing GIMP, Kodi (and optimizing it) and VLC. Mount NFS shares, connecting a bluetooth speaker, watching Disney+ while casting Netflix and messing around with the desktop environment.
Software
Today was day to install some software that I use once in a while
GIMP
Gimp was installed using apt . It works well, but it's a bit slow opening images and in some operations. Resizing and cropping work with any problems.
Kodi
I use Kodi once in a while to stream from my external NAS. Kodi is available to Raspberry PI from apt
sudo apt-get install kodi
Kodi requires AT LEAST 160MB for the graphics card. 320MB is the recommended.
I've tried to run Kodi with the default Raspberry PI OS configuration and it's really really really slow.
I've found this information in the Raspberry PI forums.
After a reboot, the difference is notable. It even speed up the performance on the desktop. Why isn't this recommended ? Or did I miss something ?
I'm not saying to 320MB if you don't need Kodi, but at least 160MB .
To have sound on Kodi, I had to change the audio output to my QCY earphones. After that, no problem whatsoever. Everything perfect.
NFS
I have a NAS that is exporting NFS shares (and samba). Mounting a NFS share is easy with Linux.
Just create a folder where to mount the share
sudo mkdir /mnt/sharing (or any name you want)
Edit the /etc/fstab file to make the share permanent and easier to mount or to mount it automatically at boot
vi /etc/fstab
add the following line:
<NFS_SERVER_IP>:</PATH> /mnt/sharing nfs defaults,user 0 0
Save and mount the folder
mount /mnt/sharing
If no error reported, it has been mounted and you can now see the shares
VLC
I also tried VLC and nothing to report. Fast and smooth video. Subtitles worked fine. This is no 5.1 doulby atmos. It's normal stereo, 1080p 30fps
Bluetooth
Because today I didn't feel to use my earphones, I've connected a bluetooth speaker and it connected in an instance. I'm still using the external dongle.
Sleep mode
For several times today, I was away from the computer. When I get back, the display was off. But I didn't configured nothing. By default it is 10 minutes . You can read the documentation. I really wanted a screensaver, so I installed . Again, this is all in the documentation.
sudo apt-get install xscreensaver
You now have a new item on the preferences menu
Disney+
Today I tried Disney+ - Falcon and Winter Soldier (Disclaimer: They belong to Disney and the following image shows one frame of the show) - and also worked, like Netflix. And, as I watched The Falcon and the Winter Soldier, I was casting Netflix to my son and everything smooth . No slowdowns on video on Disney+, no slowdowns on Netflix casting on the TV.
EDIT: At some point, Disney+ started to shudder a bit and the page even crashed...But after reloading it (Chromium didn't crash, just the tab were I was watching Disney+) it stayed smooth until the end.
This RPI 400 really can hold it.
How was the RPI 400 during this time ?
Well, for 5 minutes I had two scripts showing me the CPU frequency and temperature.
The utility vcgencmd can give some information about the Raspberry PI, and so, I issued two commands, in separate terminal windows:
/opt/vc/bin/vcgencmd measure_temp
and
/opt/vc/bin/vcgencmd measure_clock arm
So, using watch command, I've set it to execute the vcgencmd every 1s
watch -n 1 /opt/vc/bin/vcgencmd measure_temp
and
watch -n 1 /opt/vc/bin/vcgencmd measure_clock arm
The result is the following picture (Again, from The Falcon and Winter Soldier, from Disney).
The temperature never exceeded the 42 degrees celsius - but it is not summer yet in Portugal 
and speed oscillated from 1GHz to 1.8GHz
Desktop Environment
Today I came to the conclusion that I could use another virtual desktop. I search around and around and tough that if there was a possibility of another desktop (Linux has had virtual desktops for decades), it must exist a panel item to show the desktops or at least, to configure them.
Bingo
Desktop Pager to the rescue
Now, right click on the Desktop Pager, and we get a range of new options that are not readily available on the desktop menus. Why is that ?
We can now configure more options of the lxde desktop environment. Even the theme - that was something I could and will change.
To change the desktop theme, there's a small application that will do just that.
Open a terminal window and execute
lxappearance
The following window should appear
For a darker theme, for now, I've selected Adwaita-dark and the Window Border I've selected Velnias (installed in the steps above).
And a new desktop it is
You can install more using apt or just browse the internet.
You can also install icon themes.
Create two folders in your home :
mkdir .icons mkdir .themes
the dot before the name makes them hidden
Just place the icons in the .icon folder and the themes in the .themes folder. Execute again lxappearance and the new themes and icons will appear listed.
Hardware
Raspberry PI Official Mouse
After two days of using it, I like how it feels on my hand. It's just the perfect size.
Conclusions day 2
It has been a good day. Discovering how to change themes and add the screensaver has been fun. I do notice that sometimes it can get a bit slow opening Firefox and changing tabs. Navigating Netflix and Disney+ is not always smooth and fast, but I believe that's because they are trying to load the previews while the mouse pointer hovers through the shows.
Firefox is a bit slow loading some pages, but I bet is because of the version. Chromium is faster.
Tomorrow will be another day.