Add some initial docs for Themes

[zadjii-msft]

ala microsoft/terminal#3327. There's like, 8 PRs that this encompases.

More themes would be better.

[opbld30]

Docs Build status updates of commit 7128529:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/customize-settings/themes.md	✅Succeeded	View
TerminalDocs/custom-terminal-gallery/theme-gallery.md	✅Succeeded	View
TerminalDocs/images/seamless-theme-example.gif	✅Succeeded	View
TerminalDocs/images/Under-Construction-Theme.png	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[opbld31]

Docs Build status updates of commit 7128529:

✅ Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/customize-settings/themes.md	✅Succeeded	View
TerminalDocs/custom-terminal-gallery/theme-gallery.md	✅Succeeded	View
TerminalDocs/images/seamless-theme-example.gif	✅Succeeded	View
TerminalDocs/images/Under-Construction-Theme.png	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel