Wappler is a huge improvement over Bubble for UI thanks to the Bootstrap framework, especially for responsiveness. You are correct in saying that to create a unique style you need to edit CSS, which would be true for any custom website or app.
The Wappler team are improving the design panel and are adding new features over time. There is also a feature request to support more frameworks such as Tailwind here. If you have specific improvements I’m sure they would be glad to heat suggestions.
I think recently there was a discussion about adding properties such as shadows to the panel.