It is advisable to update from FA v2.3.21 to FA 2.3.26 and then choose a theme from there. If you have heavily modified your FA install and / or depend on some extensions, you may need to tweak any theme to suit your needs. If you use any language other than English, the CSS and other column/row spacing may need to be adjusted as well.
There are no DB changes worth mentioning that affect an installation for this upgrade as far as the core is concerned and no extensions that alter the core tables / files are used.
The changed files are attached herein. If you do commission anyone to write a theme for you, it will be good to share it with the community.