RGB code is not highlighted when there's floating point value

[C17AN]

Including decimal points in rgba values is not correct syntax, but I have experienced situations where both the text and background turn completely black, making it difficult to verify the input values.

For example, if you incorrectly input something like rgba(0,0,0.1), both the text and background turn black, rendering the values unreadable. (Attached at screenshot)

• Current highlight
  

• Expected highlight
  

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I have submitted a PR for this issue, and I would appreciate it if you could review it.

#209