Pluralsight is typically my go-to, namely because my work pays for my subscription and the amount of quality content is endless.
is also great for learning not only the “what”s of great UX design, but also the “why”s. Some content is better than others on that site, but there are some interesting articles and comments that will lead you in the right direction.
For bootstrap help, I’d check out bootsnip.com. They have tons of pre-built bootstrap widgets, menus, layouts, etc. that use the bootstrap library, so they are very plug and play right out of the box.
Browsing codepen.io’s top and picked pens is a great way to see beautiful ux/ui design in action and the code behind it.
Ultimately, great UI design just comes out of practice. I would suggest doing a week-long, or even 30 day challenge on codepen to push the boundaries of what you can accomplish. Really have no agenda in mind other than to try your best and see what you can learn and what you can make out of it. That experience and practice is invaluable, in my opinion. Most importantly, don’t forget to have fun with it! Try to design something YOU would be pleased to use.