As a backend developer, you are extremely weak in handling the UIUX of a web application, especially for beginners. Therefore, to quickly build a full-featured web application, you can refer to the following CSS frameworks to bring simplicity, flexibility and smooth integration with backend technologies.

1. Bootstrap: One of the most popular and widely used CSS frameworks, Bootstrap provides a robust set of pre-designed components and a responsive grid system. It's well-documented and offers a great starting point for building responsive websites.

2. Tailwind CSS: Tailwind CSS focuses on utility classes, allowing you to quickly apply styling without writing custom CSS. It's highly customizable and plays well with JavaScript frameworks.

3. Bulma: Bulma is a modern CSS framework that emphasizes simplicity and clean code. It's built on Flexbox and provides a responsive grid, making it easy to create attractive layouts.

4. Foundation: Foundation offers a responsive grid system, UI components, and a set of tools for building responsive and customizable websites. It's popular among developers who want more control over the design.

5. Semantic UI: This framework uses human-friendly HTML to create responsive and visually appealing interfaces. It provides a variety of components and theming options.

6. Materialize CSS: Based on Google's Material Design guidelines, Materialize CSS offers a set of responsive components and animations that follow a consistent visual language.

7. UIKit: UIKit is a lightweight and modular front-end framework that provides a range of components and utilities. It's designed to work well with a variety of backend technologies.

8. Pure.css: Pure.css is a minimalistic framework that focuses on small size and fast loading. It provides basic styling for common elements without imposing too much design.

9. Spectre.css: Another lightweight framework, Spectre.css, offers a set of modern and responsive CSS components that can be easily integrated into your projects.

10. Milligram: Milligram is a minimalist CSS framework that focuses on a clean and minimal design. It's great for projects where you want a simple starting point and plan to customize extensively.

Remember that the choice of CSS framework should depend on your specific project requirements, your familiarity with the framework, and how well it integrates with your backend stack. It's also a good idea to keep an eye on the latest updates and community support for each framework.