Enable alternative themes for Bootstrap (e.g. Boostwatch)

[sio]

Alchemy is a great theme, but it was not using the full potential of underlying technology. Bootstrap provides an easy way to customize look and feel of rendered pages, and a lot of themes for Bootstrap are freely available, e.g. at https://boostwatch.com

This commit introduces a new configuration variable BOOTSTRAP_CSS to make use of any existing Bootstrap skins. All incompatibilities in theme.css were fixed:

• Instead of using constant color values we use Bootstrap css variables for colors: https://getbootstrap.com/docs/4.3/getting-started/theming/#css-variables
• The only hardcoded value left is the color for header/footer borders. It was moved into a variable --alchemy-border for easier overriding if needed. I have tested current value against both dark and light Boostwatch themes, it works ok.
• Pagination template was modified slightly to avoid producing invisible text with some of Bootstrap skins

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I understand that you're using this theme on several web sites including https://nairobilug.or.ke/. After applying these changes resulting web sites will look slightly different - not all hardcoded colors could be replaced with variables in such a way that produced consistently good results across different Bootstrap skins.

There are several options:

• If new look is acceptable, no one needs to do anything and we all continue to use this great theme. Here is how nairobilug.or.ke will look with Boostwatch Yeti skin: front page screenshot
• We can add a new configuration variable to provide the overriding CSS stylesheet, and you can make use of it to maintain old looks with the updated theme. I plan to implement this feature and to submit it in a separate pull request.
• I can continue maintaining a separate fork of this theme

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I've noticed a similar pull request (#63) only when I went to submit this one. Apart from mine being newer (hence guaranteed to merge cleanly) I consider my approach slightly more generic and therefore favorable :)

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Thank you for creating this beautiful theme! :+1: There are very few good themes for Pelican!

PS: your website is so much cooler than that of my local LUG :tada:

[rwanyoike]

LTGM; we can merge this and close #63

[moorepants]

It looks like the oldstyle.css does not actually restore the prior css for some aspects. The background colors are not correct, for example.

Here is the screenshot of the default them prior to @sio's work:

https://github.com/nairobilug/pelican-alchemy/blob/8e2377f8f99ed6aabcb7e5d4d8175bf0ad5a1d01/screenshot.jpg

And here is the new one:

https://github.com/nairobilug/pelican-alchemy/blob/master/screenshots/oldstyle.png

Note that the header has a grey background and the rest of the page is white prior to this change.

[sio]

I have fixed the background issue for oldstyle.css and have updated screenshots and demo website.

@moorepants, thank you for reporting!

[moorepants]

@sio, thank you :)