well the type conversion method is not valid solution because it change objective of the problem i mean it change the type of the field which is very important the SCSS method solved my problem also we should inherit it from base as mention by @waqas also should follow the file structuring and include in manifest:           

custom_module
├── __init__.py
├── models
│   ├── __init__.py
│   └── ...
├── static
│   └── src
│       └── scss
│           └── style.scss
└── views
    ├── __init__.py
    └── assets_backend_custom.xml

Found a solution for this
create scss
 and add this 

.o_list_view .o_list_table thead > tr > th:not(.o_list_record_selector).o_list_number_th {
    text-align: center;
}
.o_list_view .o_list_table tbody > tr > td:not(.o_list_record_selector).o_list_number {
    text-align: center;
}  
and inherit template

  <template id="assets_backend_custom" name="assets_backend_custom" inherit_id="web.assets_backend" >
<xpath expr="." position="inside">
             <link rel="stylesheet" type="text/scss" href="/modulename/static/src/scss/style.scss"/>
      xpath>

  template>


this will centralize both header and fields