Thank you @arnaud for your exhaustive reply! That makes a lot of sense 
To be honest, I wouldn’t use such customization too extensively mainly because (i) streamlit’s components come with consistency built in which is something that is not trivial to achieve when designing a UI/UX from scratch and having total styling freedom in my opinion and (ii) if I were to find myself bending Streamlit’s native capabilities to serve my needs too much I would feel like I am fighting the framework instead of harnessing its power, and would therefore look to move to away from it and towards alternative ways of building my app that give me more styling freedom (e.g. using a more traditional fullstack, with something like React + UI library/tailwind in the frontend).
That being said, I do think Streamlit users would benefit a lot if widgets had default styling options at least for the “main” stuff such as font size, background color, text color, width, height to name a few. While I can see why it’s overkill to enable full customisation of Streamlit’s native components (something that not even UI component libraries do!), I also fear that being too rigid (i.e. not enabling any customisation) is also not ideal as one could build an app whose UI/UX could improve drastically with very minor changes (stuff like making the font size in the tabs a bit bigger) but is instead stuck with what they have.
For instance, in the app I am building I do change the color of the text in some places (by making it green or red depending on the kind of information it conveys for instance). Using hacky ways of achieving this kind of style (i.e. by writing CSS and setting unsafe_allow_html=True) feels like a lot of work and makes the codebase way less clean and readable in my opinion. I think many users will eventually end up using these kinds of hacky ways to implement very basic styles (things like text color or font size) anyway so why not make them native features?
Long story short, I believe a middle ground between a rigid style that can’t be customised at all (unless one uses dirty/ugly and hacky workarounds) and a full customisation would be ideal for a framework like streamlit.
Last but not least, I think streamlit is somewhat confined to the realm of data-driven apps also because of this lack of customisation. I think it would be a powerful tool to build other kinds of apps too, (such as product MVPs, internal company tools, CRUD apps and a lot more) if it enabled users to customise their apps just a little bit more.
This is just my take though! Hope this makes sense:) What do you think? And how do you see Streamlit evolve in the future?