Thank you so much for your response. I have done as you suggested, which has turned the text inside the lozenges to black, however, I am really trying to change the colour of the lozenges themselves, I need the text white and the lozenges to be a paler blue until hovered-over

Then please use the below CSS code instead of above shared.

.primary-desktop-light .sh-header:not(.sh-sticky-header-active) .c-hamburger:not(.is-active) span,
.primary-desktop-light .sh-header:not(.sh-sticky-header-active) .c-hamburger span:before,
.primary-desktop-light .sh-header:not(.sh-sticky-header-active) .c-hamburger span:after {
    background-color: #000!important;
}

.primary-desktop-light .sh-header:not(.sh-sticky-header-active) .sh-nav > li > a,
.primary-desktop-light .sh-header:not(.sh-sticky-header-active) .sh-nav > li.menu-item > a > i {
    color: #000!important;
}

.sh-portfolio-filter-style4 .sh-filter-item .sh-filter-item-content {
    background-color: #47c9e5!important
}

You can change the color value in the above code to whatever you want to use by referring the following pages.

http://www.w3schools.com/html/html_colors.asp
http://www.w3schools.com/html/html_colorvalues.asp
http://www.w3schools.com/tags/ref_colorpicker.asp

2. Thanks, I have visited the “partners” element. It allows me to allocate a URL to each logo, but not to create a page on which I might show a testimonial?

You can create page from general WordPress pages section and then link it in the partners widget.