It would certainly be very unusual if a theme you were using in RW5 would not be available to use in RW8.
Realmac kept all the RW5 themes in RW8 and they are listed under the RapidWeaver Classic Themes group, in the themes window.
To my knowledge, none of these older themes have been deleted and they do still all work. However they use an older XHTML doctype (not ideal for modern websites, HTML5 is preferred) and they are not mobile responsive.
Having said that, at times, code has been shared on these forums and elsewhere to make old themes better for mobile, so I would not rule them out as completely incompatible with mobile. And indeed, some people still prefer the approach of building separate websites for separate devices - so you might prefer to use one of these classic themes for a desktop site and a different theme for a mobile site. It does happen more often than you might think!
Third party themes like D-Scribe, again, I do not see any reason why these should not work in RW8. As @teefers correctly says, you will probably need to manually copy the theme over to RW8. This can be done by opening your RW5 addons folder (from the menu?) and navigating Finder to find the theme and any other associated files (like saved style sets). Install into RW8 by first copying them to your desktop and then dragging them onto the RW8 dock icon.
The only explanation I can think of for an older third party theme not to work in RW8 could be if the developer set a ‘flag’ by means of a key in the theme.plist file to restrict the theme outside of a certain API range. But I have never seen that done myself, so I’d say that was highly unlikely. We tend to only see this sometimes happen with stacks.
So far as CMS is concerned, how often will you be editing the website and how many people need access? If it is only one or two people occasionally needing to edit some content on an infrequent basis, then one of the free or paid CMS solutions for RapidWeaver should be suffice. If on the other hand the website needs to be edited often and by multiple people, then Wordpress or one of the other mainstream CMS publishing platforms would be a better tool for the job.