In sidebar I can select a degraded color but I cannot create a new Degraded style.
Yes creating a custom gradient in general is quite a pain as you have to open the area dialog in order to create it, and even when you do it, you dont have the option to use a custom color. Even worse is in the color, gradient, and bitmap tabs dont appear in the Format > Page dialog. Worse than that, the bitmap tab doesnt allow you to import an image into the bitmap options, which is available in writer and calc. I addressed the issue with the sidebar area section in bug 90078, integrating the new color picker in the dialogs with bug 84267, and address the issue of not having the gradient tab in the Page dialog in bug 87615.
This was fixed with the GSoC 2016 work and is available since LO 5.2.