Snippets Game Visual Rework

[ricardoboss]

Umbrella issue: #94

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The current style for the snippets game has some flaws:

• The black text on red background isn't very readable (neither is black on dark green)
• There are some grey blocks above and below the code area

A comment from earlier: https://github.com/ricardoboss/Prolangle/pull/70#issuecomment-1892886382