How do I style a div in a lighting card with css

My code in the html is:

<template>
    <lightning-card>
        <h3 slot="title">
            <svg w... /?
Some text
</h3></lightning-card>

which makes html <article class="slds-card"><div class="slds-card__header slds-card_grid">.  I need to change the padding-left value to 0, but I know I can't just add a css file with:

.slds-card .slds-card__header{padding-left: 0;}

so what's the right code?