Hi Vijaya, 

Did you have a look at this part of the documentation: https://docs.mendix.com/howto/front-end/customize-styling-new/? I think that is a good starting point for your custom styling of pages.

As a starting point, you can try to change the following variables in your project folder / them / web / custom-variables.

$brand-default
$brand-primary
$brand-success
$brand-warning
$brand-danger

These colors are used by for example buttons on your pages. After you have changed them they should be automatically injected into your browser.