You can manually change individual folder and file icons. However, the last I checked. Apple's security policies killed theme Apps like Candybar. There are system icons which cannot be changed. I don't think you are allowed to change App icons anymore either.
Mac OS 10.7 came out in 2011. The only Mac of that era which can run the latest Mac OS (
Mojave, 10.14) is the Mac Pro (with Metal compatible GPU). For any other model which shipped with Lion (10.7). The latest officially supported version is
High Sierra (10.13). If this was an older computer upgraded to Lion check the next cutoff
El Capitan (10.11) as anything which can run Sierra (10.12) can run High Sierra.
Figure out which version of macOS your Mac supports by looking up it's model (Apple Menu , About this Mac). Then you can download older macOS versions from the App Store by following the download links in the above Apple Support Articles.
Be aware sometime around 10.10/10.11. Mac OS became much more optimized for SSD. Mac with hard drives get hit with a heavy impact on performance. An SSD and 8GB RAM is recommended for 10.11 and later.