🎨 CSS Update

Update css for TOC title to avoid the duplication.

        .toc {
            background-color: var(--color-tag-bg);
            overflow-x:auto;
            padding: 0.01em 1em;
            border-radius: 10px;
            
            > ul::before {
                content: '≡文章目錄';
                font-weight: 700;
                display: inline-block;
                margin-left: -1.5em;
                margin-bottom: 0.1em;
                letter-space: 0.01em;
            }