Old template or Theme (from 3.5.1) – how do I edit it?

I have taken over a site that was on 3.5.1 up until a few months ago and I was a complete WP n00b when I took it over. It was first activated in 2006 and the original Theme was customised back during its install. I managed to export the site and import it, into a new install of 4.2 which is now upgraded to 4.3. All is reasonably OK with the site and I am no longer a complete n00b with WordPress. I have managed to install the right plugins for various bits a pieces and the site looks as good as it ever did. It also works, with all the links doing there thing etc. I am reasonably comfortable doing stuff on the linux server that the site runs on. The issue I have is I want (or require to more like) change the Theme appearance. None of the static side bar areas appear to be editable anywhere. So I can make new posts and generally alter things the way I want to but I cannot change the theme as originally customised by the previous owner of the site. I appear to be missing an editor. None of the installed editors appear to be able to “see” the bits of the pages or site that I want to change. I have Black Studio TinyMCE Widget installed and Page Builder by Site Origin, neither of which appear to be able to see the side bars etc…….any hints of what to load that might allow me to do something with this issue will be gratefully received. I am loath to simply learn HTML and the use of CSS and would much rather do something with a wysiwyg editor if one exists that would solve my problem. I appreciate that I might simply be completely ignorant of the right bit in WP that I should be using to make my changes.