fix: do not use !important as part of custom CSS property

[web-padawan]

Description

This is a finding from TaskMob where the following CSS breaks transparent background for tertiary-inline button:

:host(:not([theme~="tertiary"]):not([theme~="primary"])) {
    background-color: var(--lumo-primary-color-10pct);
}

When used as part of custom CSS property value, !important apparently gets lost.
This problem was introduced long ago in #6672 (exists in V24.3+).

Type of change

• Bugfix

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud