Fix svgbob diagrams in dark mode

[hmn53]

Closes https://github.com/boozook/mdbook-svgbob/issues/22

Currently, the SVG text elements receive the default color, black, which is not readable on dark mode.

This PR adds a one-liner CSS fix to apply the foreground css variable (var(--fg)) to the svg text element to make svg text legible and visible.