Workaround
I have chosen 48px
as the border-radius
for the below workaround. you may change this value according to your needs.
Edit the file ..../themes/Mantra/gtk-3.22/gtk.css

look for the below content:
/***********
* Dialogs *
***********/
messagedialog.background {
background-color: #FAFAFA;
}
messagedialog .titlebar {
min-height: 24px;
border-style: none;
box-shadow: inset 0 1px rgba(255, 255, 255, 0.4);
background-color: #FAFAFA;
color: rgba(0, 0, 0, 0.87);
}
messagedialog .titlebar:backdrop {
color: rgba(0, 0, 0, 0.54);
}
messagedialog.csd.background {
border-bottom-left-radius: 2px;
border-bottom-right-radius: 2px;
}
messagedialog.csd .dialog-action-area button {
padding: 8px 16px;
border-top: 1px solid rgba(0, 0, 0, 0.12);
border-radius: 0;
}
messagedialog.csd .dialog-action-area button:first-child {
border-bottom-left-radius: 2px;
}
messagedialog.csd .dialog-action-area button:last-child {
border-bottom-right-radius: 2px;
}
and edit it like below:
/***********
* Dialogs *
***********/
messagedialog.background {
background-color: #FAFAFA;
border-radius: 0 0 48px 48px;
}
messagedialog .titlebar, messagedialog.csd .titlebar {
min-height: 48px;
border-style: none;
box-shadow: inset 0 1px rgba(255, 255, 255, 0.4);
background-color: #FAFAFA;
color: rgba(0, 0, 0, 0.87);
border-radius: 48px 48px 0 0;
}
messagedialog .titlebar:backdrop {
color: rgba(0, 0, 0, 0.54);
}
messagedialog.csd .dialog-action-area button {
padding: 8px 16px;
border-top: 1px solid rgba(0, 0, 0, 0.12);
border-radius: 0;
}
messagedialog.csd .dialog-action-area button:first-child {
border-bottom-left-radius: 2px;
}
messagedialog.csd .dialog-action-area button:last-child {
border-bottom-right-radius: 2px;
}
Then go to line number around 3739 and find below content:
messagedialog.csd decoration {
border-radius: 2px;
}
Change 2px to 48px as I have chosen this size as border-radius.. you have to put as per your liking.
