Alternatives and detailed information of thunderblurred

Thunderblurred

ThunderBlurred	Calendar

$ bash -c "$(curl -fsSL https://raw.githubusercontent.com/manilarome/thunderblurred/master/install.sh)"

After the confirmation message that the theme is successfully installed, open thunderbird. You'll notice that it is still not transparent. To fix this, Open Menu > Customize > Change the theme to Dark.

It is advisible to check the script first before running it.
The script will fail if you have multiple profile directories! Make sure you only have one!
If you have a current chrome folder in your profile directory, the script will make a backup.

Open the Thunderbird Menu located on the top-right corner with a humburger menu(three horizontal lines).
Select Preferences, then Preferences again.
Go to Advanced, find the Config Editor button then press it.
A dialog will warn you, but ignore it, ~~just do it~~ press the I accept the risk! button.
Search for toolkit.legacyUserProfileCustomizations.stylesheets, layers.acceleration.force-enabled, gfx.webrender.all, and svg.context-properties.content.enabled. Make sure to enable them all!
Go to your thunderbird profile located in $HOME/.thunderbird/XXXXXXX.default-release/.
Create a folder and name it chrome, then assuming that you already clone this repo, just copy the theme to chrome folder.
Finally, change the Thunderbird theme from default to dark. This is important!

Enable the blur in your compositor. Go to System Settings > Desktop Effects > Enable Blur. Note that this will not enable the blur effect on all applications.
Enable the blur effect on all applications by installing a KWin script called Force Blur.
Go to System Settings > KWin Scripts > Enable Force Blur.