I want to change the shapes of my custom nodes like circular. rectangle etc can i change them.
No. The shape of the node in the workspace is not something you can change.
Then, how is it possible to change node styling as others were doing, apart from changing node icons
Despite you put yourself on a thread that is years old, I'll try to give an explanation:
The posts you linked refer to the visual representation of data on the Node-RED dashboard. The dashboard is kind of a "standard" web page, powered by NR flows. The elements on this page can be customized "as you like". The posts you link give you some examples of what's possible.
The look of a node in the NR editor yet is standardized. You may alter some properties (like it's color), but the editor defines the shape / appearance of the nodes in the workspace.
So using node-red editor, node customisation like shape and other styling effects are not possible
Modifications like color, icon change can be possible
There are projects based on NR that operate with differently shaped "nodes", yet this is done by replacing parts of the NR core.