Ultimately, this isn't something we can answer for you. The best approach is to ask the question of your users and get feedback about how important they feel it is. Then decide if it's worth the time and effort.
However, I would say there is one key factor that can definitely support having a "themes" feature and that is branding.
The apps you mention (facebook, linkedin, etc.) don't allow themes because for them, their colour schemes are their identity, it is very important for them that the user feels their brand when they use those apps. This is applicable for applications that are only used by the people who develop them (as opposed to sold for others to use).
Your application on the other hand, seems to be the type of application a business will buy, and then use for their own requirements. In this instance, themes can be important because it allows your customer to apply their own "branding" to the software. This becomes even more important if your software is accessible to your customer's customers, as they will no doubt what to make it look like one of their own products.
Another benefit to themes is usability. Some users may find it easier to work with certain colour schemes. For example: light mode, dark mode, high contrast mode, and themes that support colour blind requirements, etc.