LWC. Interesting approach to style standard LWC components

myComponent.css

/* Dark lightning datatable without header actions simplified example */

/* Better to create selectors with your entire component (c-my-component)  */
c-my-component lightning-datatable .slds-th__action-button {
    display: none;
}
/* Sometimes it needs to create more specific selectors as standard components may contain another componenets */
c-my-component lightning-datatable th .slds-cell-fixed {
    background-color: #333;
    color: #fff;
}
c-my-component lightning-datatable th .slds-cell-fixed:hover  {
    color: #111;
}
c-my-component lightning-datatable .slds-table tr {
    background-color: #555;
    color: #fff;
}
c-my-component lightning-datatable .slds-table tr:hover {
    color: #111;
}

myComponent.js

import { LightningElement, api, track } from "lwc";
import myComponent from "./myComponent.css";

export default class SearchResults extends LightningElement {
    
    // Adds a style element (only once)
    renderedCallBack() {
        if (!this.hasRendered) {
            const style = document.createElement("style");

            // imported myComponent contains an object with a function(t,e,s)
            // which can return a css content
            style.innerText = searchResults["0"](null, "");
            this.template
                .querySelector("lightning-datatable")
                .appendChild(style);

            this.hasRendered = true;
        }
    }
}

Appending style element it's not ideal and you can break shadow DOM encapsulation. To keep advantages of shadow DOM it is better to create selectors starting from the component itself:

c-my-component lightning-datatable .slds-table {
    /* rules... */
}