Hi thanks for reply,
I checked the source code and found the necessary files,so this is what i did:
<p>&lt;template id=&quot;assets_backend&quot; name=&quot;custom_style&quot; inherit_id=&quot;web.assets_backend&quot;&gt;</p>
<p>&nbsp; &nbsp; &nbsp; &lt;xpath expr=&quot;.&quot; position=&quot;inside&quot;&gt;</p>
<p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &lt;link rel=&quot;stylesheet type=&quot;text/css&quot; href=&quot;css file path&quot;&gt;</p>
<p>&nbsp; &nbsp; &nbsp; &nbsp;&lt;/xpath&gt;</p>
<p>&lt;t/emplate&gt;</p>
my css file :
.o_main_navbar {
position : relative;
height : 46px;
border-bottom : 1px #navbar_underline_color;
background-color : #navbar_color_desired;
color : #the_font_color;
}

<template id="assets_backend" name="custom_style" inherit_id="web.assets_backend">
<xpath expr="." position="inside">
<link rel="stylesheet" type="text/css" href="/HTC_decharge/static/src/css/custom_style_.css"/>
</xpath>
</template>