Dark/Light Theme feature added

[caronc]

Description:

Related issue (if applicable): refs #79

Added a Dark theme with the ability to toggle using icon at the top right hand side of the page.

The default mode is light, but this can be over-ridden if you set the APPRISE_DEFAULT_THEME to be dark instead.

This is entirely based on @STaRDoGG. I just made a few tweaks to some of the colors. I re-organized the inclusion of the CSS files as well (calling highlight.min.css last). in theory, this should be renamed to dark.css and another called light.css and some of the other duplication removed (especially the hacked entries in the materialize.min.css file)... But this is still a great start. Thank you so much @STaRDoGG for all your great work found here.

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [ ] Tests added

[codecov-commenter]

Codecov Report

Patch coverage is 95.65% of modified lines.

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Files Changed	Coverage
apprise_api/core/middleware/theme.py	94.73%
apprise_api/core/themes.py	100.00%

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