Fix table colors

What does this MR do?

Makes table colors darker. It fixes the issue described in #213324 (closed).

How to test?

Finding a table in GitLab’s local environment to test this component is surprisingly difficult. I took this example from BV and pasted it in a Vue component that was easily accessible:

<template>
  <div>
    <b-table striped hover :items="items"></b-table>
  </div>
</template>

<script>
  export default {
    data() {
      return {
        items: [
          { age: 40, first_name: 'Dickerson', last_name: 'Macdonald' },
          { age: 21, first_name: 'Larsen', last_name: 'Shaw' },
          { age: 89, first_name: 'Geneva', last_name: 'Wilson' },
          { age: 38, first_name: 'Jami', last_name: 'Carney' }
        ]
      }
    }
  }
</script>

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #213324 (closed)