How to style :root without !important using proper specificity - css

Question

Inside a Custom Element because border-color is set on the parent page, I can not make border-color work without resorting to !important
:host([player="O"]) {
color: var(--color2);
border-color: var(--color2) !important;
}
The selector works fine, the color is set,
so it is a Specificity issue
Question: Is it possible without !important ?
Working snipppet:
window.customElements.define('game-toes', class extends HTMLElement {
constructor() {
super();
let shadowRoot = this.attachShadow({
mode: 'open'
});
shadowRoot.innerHTML = 'Toes';
shadowRoot.appendChild(document.querySelector('#Styles').content.cloneNode(true));
}
});
:root {
--boardsize: 40vh;
--color1: green;
--color2: red;
}
game-toes {
width: var(--boardsize);
height: var(--boardsize);
border: 10px solid grey;
background: lightgrey;
display: inline-block;
}
<TEMPLATE id="Styles">
<STYLE>
:host {
display: inline-block;
font-size:2em;
}
:host([player="X"]) {
color: var(--color1);
border-color: var(--color1);
}
:host([player="O"]) {
color: var(--color2);
border-color: var(--color2) !important;
}
</STYLE>
</TEMPLATE>
<game-toes player="X"></game-toes>
<game-toes player="O"></game-toes>
qomponents

Answer 1

You are using CSS variable so you can still rely on them like this:
window.customElements.define('game-toes', class extends HTMLElement {
constructor() {
super();
let shadowRoot = this.attachShadow({
mode: 'open'
});
shadowRoot.innerHTML = 'Toes';
shadowRoot.appendChild(document.querySelector('#Styles').content.cloneNode(true));
}
});
:root {
--boardsize: 40vh;
--color1: green;
--color2: red;
}
game-toes {
width: var(--boardsize);
height: var(--boardsize);
border: 10px solid var(--playercolor,grey);
color:var(--playercolor,#000);
background: lightgrey;
display: inline-block;
}
<TEMPLATE id="Styles">
<STYLE>
:host {
display: inline-block;
font-size:2em;
}
:host([player="X"]) {
--playercolor: var(--color1);
}
:host([player="O"]) {
--playercolor: var(--color2);
}
</STYLE>
</TEMPLATE>
<game-toes player="X"></game-toes>
<game-toes player="O"></game-toes>
<game-toes ></game-toes>

Answer 2

As a complement to #Temani excellent answer: it happened because the element CSS style for <games-toes> will supersede the shadow root :host style.
According to Google's presentation:
Outside styles always win over styles defined in shadow DOM. For example, if the user writes the selector fancy-tabs { width: 500px; }, it will trump the component's rule: :host { width: 650px;}