feat: add prebuilt themes with custom properties

[yggg]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.

Short description of what this resolves:

Adds {auth,bootstrap,theme}/styles/prebuilt/components-custom-properties.scss to separate theme files from components styles. This way user could include component styles once, and then add one or more theme files that only contain theming properties settings.

Usage

[yggg]

How to use

In the styles entry of angular.json replace ./node_modules/@nebular/theme/styles/prebuilt/<theme-name>.css line with two following lines:

"./node_modules/@nebular/theme/styles/prebuilt/<theme-name>-custom-properties.css",
"./node_modules/@nebular/theme/styles/prebuilt/components-custom-properties.css",

<theme-name> is the file name of the theme you use, e.g. default or dark.

If you use multiple prebuilt themes include all themes you need and then add "./node_modules/@nebular/theme/styles/prebuilt/components-custom-properties.css". For example to enable default and dark themes add:

"./node_modules/@nebular/theme/styles/prebuilt/default-custom-properties.css",
"./node_modules/@nebular/theme/styles/prebuilt/dark-custom-properties.css",
"./node_modules/@nebular/theme/styles/prebuilt/components-custom-properties.css",