For fun I tried to make an equivalently bad-faith two-liner with vanilla HTML+CSS that preserves the Tailwind behavior:
<div class="custom-box"><div class="header"><div class="tab">app.css</div><div class="header-expand"></div></div><div class="content"><pre><code>@import "tailwindcss";</code></pre></div></div>
.custom-box {margin-top: 1.25rem; margin-bottom: 2rem; position: relative; overflow: hidden; border-radius: 0.5rem;}.custom-box:first-child {margin-top: 0;}.custom-box:last-child {margin-bottom: 0;}.header {padding-top: 0.5rem; background-color: #1E293B; box-shadow: 0 10px 15px -3px rgba(0, 0, 0, 0.1), 0 4px 6px -2px rgba(0, 0, 0, 0.05); display: flex; color: #94A3B8; font-size: 0.75rem; line-height: 1.5;}.tab {flex: none; color: #0EA5E9; border-top: transparent; border-bottom: 2px solid #0EA5E9; padding: 0.25rem 1rem; display: flex; align-items: center;}.header-expand {flex-grow: 1; background-color: #334155/50; border: 1px solid rgba(156, 163, 175, 0.3); border-radius: 0.375rem 0 0 0;}.content pre, .content code {margin: 0; background: none; box-shadow: none;}@media (prefers-color-scheme: dark) {.custom-box::after {box-shadow: inset 0 0 0 1px rgba(255, 255, 255, 0.1);}.header {background-color: #334155;}.tab {color: #bfdbfe;}.header-expand {background-color: #475569; border-color: rgba(255, 255, 255, 0.2);}}@media (min-width: 640px) {.custom-box {margin-top: 1.5rem; margin-bottom: 2.5rem;}.header {padding-top: 0.75rem;}.tab, .header-expand {padding: 0.5rem 1.25rem;}.content pre, .content code {font-size: 0.875rem;}}@media (min-width: 768px) {.custom-box {margin-top: 2rem; margin-bottom: 3rem;}.header {padding-top: 1rem;}.tab, .header-expand {padding: 0.75rem 1.5rem;}.content pre, .content code {font-size: 1rem;}}@media (min-width: 1024px) {.custom-box {margin-top: 2.5rem; margin-bottom: 3.5rem;}.header {padding-top: 1.25rem;}.tab, .header-expand {padding: 1rem 1.75rem;}.content pre, .content code {font-size: 1.125rem;}}@media (min-width: 1280px) {.custom-box {margin-top: 3rem; margin-bottom: 4rem;}.header {padding-top: 1.5rem;}.tab, .header-expand {padding: 1.25rem 2rem;}.content pre, .content code {font-size: 1.25rem;}}@media (min-width: 1536px) {.custom-box {margin-top: 3.5rem; margin-bottom: 4.5rem;}.header {padding-top: 1.75rem;}.tab, .header-expand {padding: 1.5rem 2.25rem;}.content pre, .content code {font-size: 1.375rem;}}.content > * {margin:0; box-shadow: none; background: transparent;}
(this probably an incomplete/buggy/incorrect port of Tailwind behavior)