fix(valuebox): Override theme color when default only

[gadenbuie]

For #9453

Description

Previously if a $valuebox-bg-{color} were overriden for a given bootswatch theme, e.g. "flatly", the theme override would also replace a user definition of $valuebox-bg-{color}.

See #9453 (comment) for complete details.

This doesn't completely resolve #9453 in the sense that there the user set $danger and expected value boxes with bg-danger to follow. After this PR, the recommendation for customizing value box background colors would be to directly set $valuebox-bg-danger.

For example

// instead of this
/*-- scss:defaults --*/
$danger: #009e73 !default;

// do this
/*-- scss:defaults --*/
$danger: #009e73 !default;
$valuebox-bg-danger: $danger !default;

Checklist

I have (if applicable):

• filed a contributor agreement.
• referenced the GitHub issue this PR closes
• updated the appropriate changelog
  • I'll add an item to changelog-1.5.md; can you provide guidance as to what heading to use or where to put the item?

[cscheid]

Hi @gadenbuie sorry for the delay.

• I'll add an item to changelog-1.5.md; can you provide guidance as to what heading to use or where to put the item?

## Other Fixes and Improvements seems the best.