Models and Schemas panel style adjustments

[tomaszwylezek]

Followup on #85

One hasn't been done in the previous one

• Model / Schema arrow right icon should be centered vertically, and its fill color should be configurable via --model-collapse-icon-color (fallback to #07A88D) CSS variable
• Header: there should be a border-bottom set on the whole heading and it should adjust color on hover

Current solution

Expected solution

[pk-hotovo]

PR: https://github.com/pk-hotovo/swagger-ui-kong-theme/pull/36

[tomaszwylezek]

Everything looks fine except for a border-bottom - needs a review later on

[tomaszwylezek]

Border-bottom is missing on feature feature/hotovo-kong-dev branch

[tomaszwylezek]

Checked, its done