Can’t override CSS with child theme

They are using a more specific identifier in the theme on the homepage.

try this one:

.be-section .be-row:not(:last-child) .column-block h5:last-child {
    margin-bottom: 10px !important;
}

you can change the margin to whatever you want, of course.

okay try this:

#main .be-section .be-row:not(:last-child) .column-block h5:last-child {
    margin-bottom: 5px !important;
}