Few Useful Links for Responsive Web UI Frameworks
Table of Contents
Introduction
If you want to make your website responsive for all devices, like mobile and tablet, the short answer is "it depends." However, in general, there are two approaches:
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Create responsive UI by using a responsive UI framework
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Create individual mobile or tablet apps
If you have money or time, you could do both: modify your website's UI to be responsive for other devices, and additionally create apps for individual devices. Responsive UI doesn't require as much time and money compared to creating a mobile app. However, responsive UI is not optimized for specific devices, so many device-specific functions can't be used. That's the advantage of creating a mobile app — however, it takes more time.
This blog lists a few UI framework options for creating responsive UI for your website:
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Create your own CSS and JavaScript framework
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Use Bootstrap, a popular responsive UI framework
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Use another responsive UI framework
Bootstrap-Based Frameworks
Flatstrap
http://www.flatstrap.org/download.html
A flat style based on Bootstrap.

Bootswatch
This site has dozens of different Bootstrap styles. The change is just replacing your Bootstrap.css.

One example of Bootswatch's flat style menu bar:

WrapBootstrap
https://wrapbootstrap.com/tag/mobile
This site has lots of different styles for many purposes, all based on the Bootstrap CSS framework.

One example from WrapBootstrap about a menu bar:
http://wrapbootstrap.com/preview/WB02495S4

Other CSS Frameworks
If you want to try another CSS framework, there are many out there. But that will be a big change since you'll be replacing the existing Bootstrap CSS framework.
http://mashable.com/2013/04/26/css-boilerplates-frameworks/
Foundation

Gumby

Other Resources
Responsive CSS Framework Comparison
http://responsive.vermilion.com/compare.php
Conclusion
There are many responsive UI frameworks available, ranging from Bootstrap-based themes like Flatstrap, Bootswatch, and WrapBootstrap to alternative frameworks like Foundation and Gumby. Choose the one that best fits your project's needs and design requirements.