In the Bottom Left is a navigation widget made with Page Buttons. In the picture, the "Digster" page is opened.

My question is:
Is there a way to highlight the background of each button when the particular page is loaded on screen?
For example, in this cases, the "Digester" button is clicked and "Digester" button is opened. So the Page button should have a different background color.

How to achieve this? Thank you for your time..

(somewhere put a condition like this? if page headline is 'digester' then change the color of the page button which have 'digester' written )

You can create a parameter for your pages widget called something like drawnPageTitle. Then each time you draw the widget, you can set the argument to an expression with \Title for the raw title or \GetPhrase(\Title) for the common name for each page title. To test this, create an text expression and set it to \drawnPageTitle. If it is working you should see the name of the page each widget is drawn.

From there, you can use those names to manipulate your navigation widget as you like.

Ie: create an opacity expression such as

PickValid(\GetPhrase(drawnPageTitle) == "Overview" ? 1 : 0  ,0)

Maybe the Frame Tabs / Frame Buttons / System Tabs widget would help as well? It is under Main/Home --> Decorations --> Folders and there are 3 styles that you can change the options for colours to show which one is currently selected.