Hi  Nahid,

You need to extend the "web.FormStatusIndicator" template and remove the 'btn-light' class and add the 'btn-primary' class.
referee 

follow the code written below:

xml version="1.0" encoding="utf-8"?>
id="template" xml:space="preserve">
t-name="your_module_name.FormStatusIndicator" t-inherit="web.FormStatusIndicator" t-inherit-mode="extension">
expr="//button[hasclass('o_form_button_save')]" position="replace">
type="button"
class="o_form_button_save btn btn-primary py-0"
t-att-disabled="props.isDisabled"
data-hotkey="s"
t-on-click.stop="save"
data-tooltip="Save manually"
aria-label="Save manually">
class="fa fa-cloud-upload fa-fw" />Save
                

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Brain Station 23 Ltd. 

Mobile: (+880) 1404055226 

Email: sales@brainstation-23.com 

Web: https://brainstation-23.com/ 

Address: (Building-1) 8th Floor, 2 Bir Uttam AK Khandakar Road, Mohakhali C/A, Dhaka 1212, Bangladesh 

Hi,

This filed class name is o_form_button_save
Try this code:
o_form_button_save{
  /* Increase button size */
  font-size: 16px;
  padding: 10px 20px;
  /* Change button color */
  background-color: #008000;
  color: #fff;
  /* Add border */
  border: 1px solid #ddd;
  /* Additional styles as needed */
}

Hope it helps