Add white background to SVGs, to fix invisibility in dark mode

[lucasluitjes]

What kind of change does this PR introduce? (check at least one)

• Bugfix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Code style update
• Refactor (refactoring or adding test which isn't a fix or add a feature)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Did you test your solution?

• I lightly tested it in one browser
• I deeply tested it in several browsers
• I wrote tests around it (unit tests, integration tests, E2E tests)

Problem Description

As mentioned in #85, the SVG images are invisible in dark mode. This is because they use a black foreground on a transparent background.

Solution Description

The \bg instruction does not work with SVG in codecogs, but \colorbox does. This PR wraps each expression in a \colorbox. So x = 2kj becomes \colorbox{white}{$x = 2kj$}.

For future reference, I used a sed oneliner to apply this to all instances: sed -i -E 's|\]\(http://latex\.codecogs\.com/svg\.latex\?([^)]*)\)|](http://latex.codecogs.com/svg.latex?%5Ccolorbox%7Bwhite%7D%7B%24\1%24%7D)|g' input.md

Side Effects, Risks, Impact

The margin around the expression is slightly increased (in firefox at least). This fixes the dark mode legibility problem that @mwmwmw reported in #86, but in light mode it just means the margins are slightly bigger.

[lucasluitjes]

(Description edited to include the PR template and mention the difference between this PR and the other dark mode PR)