Fix CSS deprecations

[RichardLitt]

Deprecation Warning: darken() is deprecated. Suggestions:

color.scale($color, $lightness: -41.1290322581%)
color.adjust($color, $lightness: -20%)

More info: https://sass-lang.com/d/color-functions

   ╷
19 │             color: darken($teal, 20%);
   │                    ^^^^^^^^^^^^^^^^^^
   ╵
    components/_background.scss 19:20                               @import
    /Users/richard/src/sustainers/website/assets/css/main.scss 6:9  root stylesheet
Deprecation Warning: lighten() is deprecated. Suggestions:

color.scale($color, $lightness: 20%)
color.adjust($color, $lightness: 20%)

More info: https://sass-lang.com/d/color-functions

  ╷
4 │         background-color: lighten($black, 20%);
  │                           ^^^^^^^^^^^^^^^^^^^^
  ╵
    components/_button.scss 4:27                                    @import
    /Users/richard/src/sustainers/website/assets/css/main.scss 7:9  root stylesheet
Deprecation Warning: darken() is deprecated. Suggestions:

color.scale($color, $lightness: -20.564516129%)
color.adjust($color, $lightness: -10%)

More info: https://sass-lang.com/d/color-functions

  ╷
6 │         color: darken($teal, 10%);
  │                ^^^^^^^^^^^^^^^^^^
  ╵
    components/_button.scss 6:16                                    @import
    /Users/richard/src/sustainers/website/assets/css/main.scss 7:9  root stylesheet
Deprecation Warning: lighten() is deprecated. Suggestions:

color.scale($color, $lightness: 13.1443298969%)
color.adjust($color, $lightness: 10%)

More info: https://sass-lang.com/d/color-functions

  ╷
2 │     border-bottom: 1px solid lighten($darkblue, 10%);
  │                              ^^^^^^^^^^^^^^^^^^^^^^^
  ╵
    components/_navbar.scss 2:30                                     @import
    /Users/richard/src/sustainers/website/assets/css/main.scss 14:9  root stylesheet
                    done in 1.432 seconds.

Came up while running the build.