thank you for help but its not working as it should be.
i have created .css file and added entry in __manifes__.py file as below, restart server, upgraded module and navigate to the page in my test module but there is not change in priority widget. my be i did something wrong. please advice.

om_hospital/static/assets/mydeco.css:

.o_field_widget .o_priority > .o_priority_star.fa-star {
color: #008000;
}

in __manifest__.py :

'assets': {
'web.assets_backend': [
'om_hospital/static/assets/mydeco.css',
],
},

regards

it works perfectly when add !important as below in mydeco.css file:
.o_field_widget .o_priority > .o_priority_star.fa-star {
color: #008000 !important;
}