Add fixed classes to `slider` to allow input styles to be themed

[tomdye]

Enhancement

Currently you cannot easily theme the slider without having to replicate a large number of the structural styles. For instance if you wish to remove the outline on the input you must either use an input selector which goes against the css-modules guidelines or re-implement all of the base .input class styles.

[eheasley]

We confirmed this was fixed with our latest theme updates.