in "main_layout.css" file, I wrote like this.
a {
text-decoration: none;
}
.title_bar {
border-bottom: 1px solid gray;
cursor: pointer;
padding-bottom: 10px;
}
.title_bar #title {
display: inline;
font-size: 45px;
text-align: center;
vertical-align: bottom;
}
.title_bar img {
vertical-align: bottom;
}
.main_layout {
display: grid;
grid-template-columns: 150px 1fr;
}
.left_menubar {
border-right: 1px solid gray;
}
.main_content {
padding: 16px;
}
.main_content #pg_welcome {
margin-top: 0;
}
.main_content iframe {
margin-top: 5px;
margin-bottom: 5px;
}
and text-decoration: none
doesn't work.
so I wrote the code in HTML like this.
<link rel="stylesheet" href="css/main_layout.css">
<style>
a {
text-decoration: none;
}
</style>
then it works! I don't know why it works.. and I can't find the reason on google.