theming: tab.activeBorderTop is not applied properly

[vince-fugnitto]

Bug Description:

The theming option tab.activeBorderTop is not applied properly (notice the missing green border).

theia:

vscode:

Steps to Reproduce:

1. include the following test theme (active-border)
2. start the application
3. trigger the command Preferences: Color Theme and select Active Border
4. open a tab and notice that the border is not applied properly

[vince-fugnitto]

The styling does exist but does not seem to work (box-shadow with inset):

theia-main-content-panel .p-TabBar .p-TabBar-tab.p-mod-current {
    color: var(--theia-tab-activeForeground);
    background: #1e1e1e;
    box-shadow: 0 1px 0 #a2ff00, 0 -1px 0 transparent inset;
}