e621 style tweaks

[ScoreUnder]

This PR:

• Tweaks default colours in e621 stylesheet to use the new qproperty stuff as intended (not just the darkmode defaults that were put in there as a placeholder)
• Adds a new stylesheet which goes a bit further in trying to mimic the style of the site
• Contains some small widget-painting changes that enable the background of certain custom widgets to be transparent and/or selected by QSS.

That list of changes also roughly corresponds to the three commits contained in this PR, so the scarier one with code changes can be left out if need be.

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Original e621 stylesheet if you haven't added default palette colours

Tweaked e621 stylesheet (just putting recommended defaults into the right place)

New 'reduxed' stylesheet (I haven't replaced the old one because I expect this to be more fragile in its design)

Tag manager pre-'redux':

Tag manager post-'redux':

If you leave out the changes to how transparency works, everything looks a little flatter and the rounded corners on the top right and bottom right of the Page don't draw correctly, but otherwise it's mostly fine: