[css] 表格优化：线条加粗、加深表头、强化每行背景色的对比

效果如图所示：

/* 边框加粗 */
:is(.b3-typography, .protyle-wysiwyg) table :is(th, td, tr) {
    border: 2px solid var(--b3-theme-on-background) !important;
}
/* 强化背景色 */
:is(.b3-typography, .protyle-wysiwyg) table th {
    background-color: rgba(0, 0, 0, 0.2);
}
:is(.b3-typography, .protyle-wysiwyg) table tr:nth-child(odd) td {
    background-color: rgba(0, 0, 0, 0);
}
:is(.b3-typography, .protyle-wysiwyg) table tr:nth-child(even) td {
    background-color: rgba(0, 0, 0, 0.05);
}
[data-theme-mode="dark"] {
    & :is(.b3-typography, .protyle-wysiwyg) table th {
        background-color: rgba(255, 255, 255, 0.15);
    }
    & :is(.b3-typography, .protyle-wysiwyg) table tr:nth-child(odd) td {
        background-color: rgba(0, 0, 0, 0);
    }
    & :is(.b3-typography, .protyle-wysiwyg) table tr:nth-child(even) td {
        background-color: rgba(255, 255, 255, 0.05);
    }
}