material.min.css should replace
bootstrap.min.css, not applied over it.
bootstrap.min.js can be used as it is while
material.min.js, optionally, can be added as an addition for some extra functionalities such as floating labels and ripple effects.
Getting back to your question, you can update
browsers settings of the
postcss task in
Gruntfile.js (around line 162) to include IE 8, so when you recompile your CSS, the
postcss task will try its best to generate CSS that can support IE 8. However, I doubt this will be sufficient mainly due to IE 8's lack of support for flexbox, media queries, etc as these are used regularly in this project. You will need to include some poylfills for the CSS to actually work in IE 8. A good place to start is:
Alternatively, we do have a version that is built upon Bootstrap 3 and this is currently sitting in the
maintenance branch. Although this version does not support as many features as the currently active branches (dev, master), it may suit your need.