Tweak a Wordpress theme design

We are currently working on a new website design for a magazine site, with the new site being built on a test server. The chosen theme has already been uploaded to the new site and some customisation carried out but it still needs some other tweaks to the CSS and layout, for example matching the colour scheme to company colours and some other design features such as background image so it is consistent with the design of the existing magazine site which is still live. Also one or two other things need tweaking, such as the alignment of an advertising widget box and mailing list submission form.