I am trying to learn how to do gradients like shown in the image. Dark color on the outside 4 corners and lighter toward the center.
There are a lot of ways to create a gradient that's dark in the corners and lighter in the center. Here's one that can help create the more organic/irregular background with the darker corners:
As Myra suggests with her opening statement, this is something that can have multiple approaches. More than most Photoshop tasks I would say.
I'm guessing that you know that the gradient tool has several options including radial.
The problem with that is that it can not do an elliptical gradient, only round, but you can start with additional canvas size and Free Transform it to the required flatter shape. There would still be an issue in that the vertical gradient would be crushed, but it is still usable
Another approach would be to fill a layer with the darker blue.
Fill an elliptical seleection with the lighter blue on a new layer, and use Gaussian blur to produce the gradient.
Note that I made the lighter blue layer a Smart Object so you can fine tune the blur amount
I liked Myra's Layer Style solution as it is fully nondestructive. You can rarely go wrong with layer styles, although some of them are finicky to use.
Here's yet another way.
Fill the area with a solid color that is the lighter color you want for the center.
Choose Filter > Camera Raw Filter.
Under Effects, move the Vignette slider to the left.
(If you want a mottled look in the center, you could set foreground/background colors to lighter blue and medium blue, then use Filter > Render > Clouds. You may want to use the Blur filter to soften the look of the clouds. Then proceed to the Camera Raw filter.)
That's good one Barbara. There are some hidden gems in Camera RAW like vignette. Add grain is one of the best fractal noise generators I know of, and a great way to make touched up skin look like real skin. I still like to recommend the Michael Ninness Photoshop Power Shortcuts from way back in MAX 2014. It is not 100% relevant because of changes, but most of it still works. At the time it was a bit of a game changeer for me.