add theme generator script

[samiebee43]

What's been done

• Given a color of choice, theme generator creates a new folder in enlighten/assets/stylesheets where Enlighten is tinted to that color

What's yet to be done

• enlighten/assets/stylesheets normally contains light and dark themes only. Enlighten has a mechanism for selecting between these two themes. I'm able to preview my theme by temporarily overwriting one of them (dark). For these themes to be selectable, we need a more generic theme selection option.
• I would like to generate 5-10 themes as starting points with this, such as yellow, blue, orange (for halloween), xtra dark, etc.

How it works (for fun)

The script copies a whole folder to make a complete theme but it only modifies enlighten.css, which drives at least 95% of Enlighten's colors. I use a HSL-based color find-replace to replace shades of gray from the light theme with shades of a particular target color. This works on embedded HSL values in the stylesheet as well as short and long hex codes (ex: #FFF, #FF0000).

There are a few colors that don't get replaced, such as the Raman button. So theme gen can be used to create a starting point for themes. It may require some additional tweaking to get it just right.

I might want to do a Halloween theme which consists of mostly dark colors with a couple of orange accents added in after the fact

[samiebee43]

Ready for review.

• A theme with a name starting with "dark" will trigger the dark graphs. (For example "dark", "darkblue", "darker")
• Contains a tentative fix to #324 copied from Mark's branch
• Fail gracefully if a selected theme is removed from disk
• Integrate with existing light/mode dark logic. (Most themes are considered light themes.)

To Do (post merge)

• fully resolve #108 when switching to and from light and dark themes
• use darker ranges of lightness in colorgen for a theme labeled "dark..." (improve appearance of darkblue theme) Done!
• themed darkmode toggle*

*explanation of "themed darkmode toggle"

Say you had the pink theme selected. With this revision, if you click the dark mode icon (moon) then it will switch to dark. If you click it again it will switch to light. It would be better if toggling twice takes you back to the theme you were on, and if the user can customize (and save) a preference of both light and dark themes.

For example I might use pink theme as my light mode and darkblue as my dark mode. This addition isn't worthwhile until the modified "dark..." themes have their color logic adjusted to actually be darker.

[samiebee43]

Added more support for custom dark themes: