Fix border bottom CSS var style

[benri]

Fixes border property overriding the style of border-bottom when the CSS variable is undefined

Fixes issue introduced by this commit ded3611#diff-eccc2b8fef024888c50f0586687e68415115c3cc38bd9fda26ba1d4cbd0ce0dcR20

[deiteris]

@benri, what's the initial idea for border styling actually? To make it possible to modify the entire border-bottom property, or being able to also modify the color of all border sides? If it's about sides, it can be done in the following way:

border: var(--api-body-document-title-border, 1px solid);
border-color: var(--api-body-document-title-border-color-full, transparent var(--api-parameters-document-title-border-color, #e5e5e5) transparent transparent);

This way the default behavior is preserved while customization of border becomes more flexible:

1. The border is set to 1px solid if variable is not defined and we can also modify the appearance of the border.
2. The border color is set to transparent for all border sides except bottom by default. It can be modified in any way we want. F.e., I can set --api-body-document-title-border-color-full to #FFF #FFF, and this will paint top and bottom borders white.

Notice that I named variable as --api-body-document-title-border-color-full, because there's a variable name clash. I am not sure how it should be handled in this case but maybe other contributors know better.

[benri]

For my use case, I am only styling the border-bottom (actually, hiding it by setting border-bottom: 0) so I may not need to color the other border sides

[jarrodek]

@deiteris @benri I am working on a consolidation of all documentation components under a single repository. You can see the new code base here: https://github.com/advanced-rest-client/api-documentation/tree/release/7.0.0
It is not yet ready as I am still merging test cases from the old components. This will be a breaking change anyway so it will be easier to make major styling changes out there. I'll be happy to hear your feedback.

[deiteris]

@jarrodek since you're moving some stuff from api-type-document there as well, please also take a look at the issues and PRs here:
https://github.com/advanced-rest-client/api-type-document/issues