Modified code snippets' theme color palette to achieve better contrast

[jasongao97]

This pull request modifies the colors used in our code snippet highlighting to ensure a minimum color contrast ratio of 4.5:1 for better accessibility. The specific changes are as follows:

1. Keyword Colors:

.hljs-doctag,
.hljs-keyword,
.hljs-meta .hljs-keyword,
.hljs-template-tag,
.hljs-template-variable,
.hljs-type,
.hljs-variable.language_ {
  /* prettylights-syntax-keyword */
  /* color: #d73a49; */
  color: #c32837;
}

2. Built-in and Symbol Colors:

.hljs-built_in,
.hljs-symbol {
  /* prettylights-syntax-variable */
  /* color: #e36209; */
  color: #aa4908;
}

3. Comment and Formula Colors:

.hljs-comment,
.hljs-code,
.hljs-formula {
  /* prettylights-syntax-comment */
  /* color: #6a737d; */
  color: #606771;
}

4. Name, Quote, and Selector Tag Colors:

.hljs-name,
.hljs-quote,
.hljs-selector-tag,
.hljs-selector-pseudo {
  /* prettylights-syntax-entity-tag */
  /* color: #22863a; */
  color: #1e7631;
}

These changes have been tested using the WAVE Evaluation tools.

It will resolve #951

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[shiffman]

This looks great! @tuantinghuang if you want to double-check it please go ahead and then I will merge!

[shiffman]

Going to go ahead and merge this we can always adjust later!