Template:Filter/styles.css

.mw-ui-button {
    border-radius: 4px;
    margin: 2px;
    color: var(--page-text-color) !important;
    background-color: var(--theme-color-2) !important;
    border: 1px solid var(--theme-border-color-subtle) !important; 
}

.mw-ui-button-group > *:first-child, .mw-ui-button-group > *:last-child {
    border-radius: 4px;
}

.mw-ui-button:not(.mw-ui-icon-element) {
    min-width: 30px;
}

.mw-ui-button-group > *:not(:first-child) {
    border-left: 1px solid var(--theme-border-color-subtle) !important;
}

.mw-ui-button-group .filter-button-selected, .filter-button-selected:hover {
    background: var(--theme-link-color) !important;
    color: var(--page-background-color) !important;
}

.mw-ui-button:not(.mw-ui-icon-element) {
    padding: 5px 10px;
}