To avoid and fix Sass and CSS issues in your Ext JS apps, you should follow some best practices. This includes utilizing consistent naming conventions and code formatting for your Sass and CSS files and variables, as well as taking advantage of Sass features such as mixins, functions, and nesting. Additionally, you should use Ext JS variables and mixins to customize your app theme and leverage its built-in features. Media queries and responsive design techniques should also be used to make your app adaptable to different screen sizes and devices. Furthermore, relative units like em, rem, and % should be used for font sizes, margins, and paddings instead of absolute units like px. Furthermore, CSS properties such as flexbox, grid, and transform should be used for layout and alignment instead of floats, margins, and position. Additionally, CSS properties such as transition, animation, and filter should be used for visual effects and interactions instead of JavaScript. Finally, web fonts, SVG icons, and sprites should be used instead of images for icons and graphics. By following these best practices, you can improve your Sass and CSS code quality, readability, maintainability, and performance.