Blur Mod 1.16.5/1.15.2/1.12.2 releases a Gaussian blur shader to the background of every GUI. It also consists of other remarkable features that you can find here for free.

All about Blur
It adds a shader to make that background fade and give it a smooth & glossy look.
It’s highly configurable. Therefore, it’s possible for the user to adjust the outer layer of the mod as you expect.
Versions for 1.14 snapshots will require Fabric.
Configuration
In fact, you can configure the strength of the blur in the config file with the radius option.
It’s feasible to reset the background colors and gradient. The default is black with 1 45% alpha value (75000000).
fadeTime is the time it takes in milliseconds.
guiExclusions is a list of classes that are not included in the blur shader.
Blur is a cosmetic mod. It’s necessary for you to download and install Minecraft Forge if you want to access it!
Screenshots