Fix small issues on RKE Template diff modal

[nwmac]

Addresses rancher/dashboard#5954

QA re-opened this issue asking for a few things:

1. Contrast is not compatible w/ Dark Mode - [unable to clearly view values]
2. Compare does NOT show two templates side-by-side, confusion around which color [red/green] represents which Template (requesting colored Template labels perhaps? or side-by-side comparison)
3. All fields with differences should be either red or green with values being compared from the respective Templates.
4. Currently, if null/nil, in first template, and populated in second template, compare will only show populated values in green, associated with the second template. [e.g. private_registries field from screenshot]

This PR addresses 1 and 2.

The other two are not easily addressed with the library we use - having reviewed the UX, with the changes in this PR, I believe this delivers an acceptable UX.

Key added:

Dark mode fixed: