Hi Rahul,
Personally, I would recommend creating a child theme based on Boost so that you can maximise your styling options:
https://docs.moodle.org/dev/Creating_a_theme_based_on_boost
Alternatively, you could add styling in the Boost theme using a preset file (
https://docs.moodle.org/dev/Boost_Presets) or direct via settings (Site Administration > Appearance > Boost (Theme) > Advanced Options.
You'd probably want to add something similar to this to the Raw SCSS section so that it is added at the end and supersedes the default styling:
header#page-header {
background-color: transparent;
background-image: url("url_to_my_hosted_wavey_header.png");
background-size: cover;
background-position: center top;
margin-bottom: 16px;
}
header#page-header .card {
background: transparent;
border :none;
}
I'm note sure what you mean by grey area? - Do you mean the border?
I hope this helps,
Anthony