fix(tabs): replace igx-icon with igc-icon in WC demo + fix theme parameters - 20.1.x

[adrianptrv]

Closes 1915
Related WC PR: #1929

• Replaced igx-icon with igc-icon in the WC Tabs demo.
• Fixed the $border-radius theme parameter so it works correctly.
• Updated $item-background to apply only to Tab items instead of both Tab items and the Tab Header.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[desig9stein]

THEME: INDIGO
It looks like item-active-background is not used anywhere in the Indigo theme. Its current value also seems incorrect: when I apply it in the expected place, it renders as transparent. I believe it should use the same color as the item-hover-background, since that’s what is currently used in that state.