Add this code to the Custom CSS box

header#header [href*="strava"] {
    background-image: url(https://static1.squarespace.com/static/644f2654b9c076768430cf4b/t/6634f146c8438a4c944dc65f/1714745670644/4691462_strava_icon.png) !important;
    background-size: contain;
    background-repeat: no-repeat;
}
header#header [href*="strava"] svg {
    display: none;
}

Result:

Before:

After: