Ideally I like having something to start from, Semantic UI providing in my experience the most flexible framework when combining different UI elements. When I need some control/widget that's not covered by the framework (did this for Bootstrap too) I build it in such a way that it fits with the way the framework works and feels as much as possible. I generally stick with the default theme of the UI framework unless a design is handed to me and try just to follow best practices for accessibility and keep it clean.
Tip for forms I found out that more often than not I prefer tooltips for displaying validation messages, because unless it's the most boring example, fields often end up in different widgets/tables/containers that are sensitive to positioning and size. So if you end up making custom forms in these kinds of requirements keep that in mind.