I am using bootstrap, tweaked to look more different. With some custom CSS, you can basically tweak any component to look more different. I usually target the CSS attributes that make the biggest difference: border, border-radius, box-shadow, background-color, font-size, font-weight, font-family (set once and forget), padding. These make the most difference. Other than that, I take my inspiration from other startups and companies. When I encounter a UI that feels and looks good, I look around the code to see what are those elements that feel good and how they are styled. I don't copy their designs but just look around to see what properties they focus on and adapt them to my own theme.