Add Nord Theme. Closes #1158 and supersedes #1159 (#1899).

* Re-add tpbnick's Nord theme

* Run Prettier formatter on Nord theme

* Update themes index

* Fix button margins

* Modernise the look of switches

* Adjust margins and padding

* Fix sidebar's background colour not applying to all of sidebar when scrolling down

* Adjust App Bar box shadow

* Adjust roundedness

* Adjust shadows

* Adjust outlined inputs

* Add transitions to items in sidebar when hovered / losing hover

* Adjust border radiuses

* Adjust pagination buttons

* Add big play button from Spotify theme

* Remove playlist background gradient

* Adjust colour of MuiChip elelments

* Adjust table borders

* Remove duplicate MuiTableRow key

* Attempt to make switches in both the playlist section and settings section visable against background & the toggle. Not ideal.

* Style the player

* Format CSS to Prettier standards

* Fix mobile player style

* Make play button in album grid view blue

* Make main view background lighter

ui/src/themes/index.js

@@ -6,6 +6,7 @@ import SpotifyTheme from './spotify'
 import LigeraTheme from './ligera'
 import MonokaiTheme from './monokai'
 import ElectricPurpleTheme from './electricPurple'
+import NordTheme from './nord'
 
 export default {
   // Classic default themes
@@ -18,5 +19,6 @@ export default {
   GreenTheme,
   LigeraTheme,
   MonokaiTheme,
+  NordTheme,
   SpotifyTheme,
 }