BigCommerce Design Help

We've recently started building our new site with BigCommerce; using a theme called Minimal by Template Mela. Unfortunately, we require a few amendments to the theme that are our of our html/CCS skill level; and Template Mela do not offer any customisation help. We have has some issues with this theme, as the theme creator has used a lot of non-scaling code. The work we require doing is as follows: Main template: • We would like the background of the site to be the same as our current website ([login to view URL]), so that the body of website stands out from the background, and remain responsive. • We would like the dropdown category lists to be justified, rather than centred • We would like a search bar fixed into the header, as it is on our current website, rather than a click at the top of the page Home Page: • We would like the banner images underneath the homepage carousel to respond properly when the page size is changed. As you can see the images go into a staircase style position when the browser size is altered. If possible, we would like to keep these fixed under the carousel. Category Pages: • The website features 300+ categories, and these are currently all featuring down the side on all category pages. If possible, could the side category nodes be collapsible and only feature the categories from the top level category (Men’s, Women’s, Jumpers, etc). • Sister categories are currently featured underneath the category heading. We would prefer these to be underneath the products