Add SDG background color classes

[jenny-alexander]

Fixes #4344

What changes did you make and why did you make them ?

WHAT CHANGES:

• In the _sass/elements, create a new file: _background-sdg-colors.scss
• In the _background-sdg-colors.scss file, I added the following:

.background-color-sdg2 { background-color: $color-sdg2; }
.background-color-sdg3 { background-color: $color-sdg3; }
.background-color-sdg4 { background-color: $color-sdg4; }
.background-color-sdg5 { background-color: $color-sdg5; }
.background-color-sdg8 { background-color: $color-sdg8; }
.background-color-sdg9 { background-color: $color-sdg9; }
.background-color-sdg10 { background-color: $color-sdg10; }
.background-color-sdg11 { background-color: $color-sdg11; }
.background-color-sdg13 { background-color: $color-sdg13; }
.background-color-sdg16 { background-color: $color-sdg16; }
.background-color-sdg17 { background-color: $color-sdg17; }

-In the _sass/main.scss, under the Elements section, add the following import statement under the Elements section
@import 'elements/background-sdg-colors';

WHY WERE CHANGES MADE:
These changes were made so we can use these classes with their corresponding, matching SDG icons.

Screenshots of Proposed Changes Of The Website (if any, please do not screen shot code changes)

Only scss file was created and imported. Code changes made had no impact on look of website.

[github-actions]

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From your project repository, check out a new branch and test the changes.

git checkout -b jenny-alexander-add-sdg-background-color-classes-4344 gh-pages
git pull https://github.com/jenny-alexander/website.git add-sdg-background-color-classes-4344

[one2code]

ETA: 5/21/23

[one2code]

The file name, contents, and file path are correct, and the import file in _sass/main.scss is named and placed under the Elements header, as identified in the action items for this issue.

Everything looks good. Congratulations on a job well done!

[t-will-gillis]

Hey @jenny-alexander - good work! Your branches look good, you are linked to the issue, your description tells what you did and why, and you noted that there are no visual changes to the website (until later issues reference these colors). Thanks for taking care of this-