:root { color: #172033; background: #f3f6fb; font-family: Inter, "PingFang SC", "Microsoft YaHei", sans-serif; }
* { box-sizing: border-box; }
body { margin: 0; min-width: 320px; }
.course-app { max-width: 1600px; min-height: 100vh; margin: 0 auto; padding: 22px 28px; }
.course-header { padding: 0 2px 18px; }
.course-topbar { display: flex; align-items: center; justify-content: space-between; gap: 24px; }
.course-header h1 { margin: 19px 0 0; font-size: clamp(25px, 4vw, 36px); letter-spacing: -.03em; }
.back-link { color: #3068e8; font-weight: 700; text-decoration: none; }
.download-link { flex: 0 0 auto; border: 1px solid #cddafa; border-radius: 12px; padding: 10px 14px; color: #255dd7; background: #fff; font-weight: 700; text-decoration: none; }
.download-link:hover { border-color: #3975f0; background: #f4f7ff; }
.course-shell { display: grid; grid-template-columns: 250px minmax(0, 1fr); min-height: calc(100vh - 142px); overflow: hidden; border: 1px solid #dfe7f4; border-radius: 24px; background: #fff; box-shadow: 0 16px 38px rgba(34,58,95,.10); }
.course-sidebar { max-height: calc(100vh - 142px); overflow: auto; border-right: 1px solid #e4eaf4; background: #fbfcff; }
.course-sidebar-head { display: flex; justify-content: space-between; align-items: center; padding: 20px 18px 14px; color: #1c3159; }
.course-sidebar-head span { color: #7a88a2; font-size: 12px; }
.course-tree-chapter { padding: 0 9px 12px; }
.course-tree-chapter h2 { margin: 11px 8px 7px; color: #4b5d7d; font-size: 14px; line-height: 1.4; }
.course-tree-lessons { display: grid; gap: 3px; }
.course-tree-item { display: flex; align-items: flex-start; width: 100%; gap: 8px; border: 0; border-radius: 10px; padding: 8px 8px; color: #33415d; background: transparent; text-align: left; font: inherit; font-size: 13px; line-height: 1.45; cursor: pointer; }
.course-tree-item span { flex: 0 0 auto; color: #6e85b8; font-weight: 800; }
.course-tree-item:hover { background: #edf3ff; }
.course-tree-item.is-active { color: #1749b8; background: #e8f0ff; font-weight: 700; }
.course-tree-item.is-active span { color: #2866e5; }
.course-reader { max-height: calc(100vh - 142px); overflow: auto; padding: 42px clamp(28px,5vw,76px) 36px; }
.course-kicker { margin: 0 0 14px; color: #4d7fe9; font-size: 14px; font-weight: 800; }
.course-reader h2 { margin: 0 0 30px; color: #17284a; font-size: clamp(26px,3.2vw,38px); line-height: 1.3; }
.course-article { max-width: 980px; color: #34425e; font-size: 17px; line-height: 1.95; }
.course-article > p { margin: 0 0 20px; }
.course-section-title { margin: 38px 0 16px; color: #1e549d; font-size: 21px; line-height: 1.45; }
.course-template { margin: 18px 0 28px; padding: 18px 22px; border: 1px solid #d5e1f2; border-left: 4px solid #4f8bea; border-radius: 12px; background: #f8fbff; }
.course-template p { margin: 0; color: #314569; line-height: 1.8; }
.course-template p + p { margin-top: 6px; }
.course-list { margin: 0 0 22px; padding-left: 1.4em; }
.course-list li { margin: 5px 0; padding-left: 4px; }
.course-flow { margin: 2px 0; color: #326ae6; text-align: center; font-weight: 800; }
.course-pager { display: flex; justify-content: space-between; gap: 12px; max-width: 980px; margin-top: 50px; padding-top: 24px; border-top: 1px solid #e4eaf4; }
.course-pager button { border: 1px solid #cfdcf5; border-radius: 10px; padding: 10px 14px; color: #255dd7; background: #f8faff; font: inherit; font-weight: 700; cursor: pointer; }
.course-pager button:disabled { color: #a7b1c3; border-color: #e2e6ed; background: #fafafa; cursor: not-allowed; }
.course-loading, .course-error { color: #62718c; font-size: 17px; }
@media (max-width:760px) { .course-app { padding:18px 14px; } .course-header { padding-bottom:16px; } .course-header h1 { margin-top:15px; } .download-link { padding:9px 11px; font-size:13px; } .course-shell { display:block; border-radius:18px; } .course-sidebar { max-height:230px; border-right:0; border-bottom:1px solid #e4eaf4; } .course-sidebar-head { padding:16px; } .course-tree { display:grid; gap:2px; padding:0 8px 14px; } .course-tree-chapter { padding:0; } .course-tree-chapter h2 { margin:10px 8px 5px; } .course-tree-item { padding:7px 8px; } .course-reader { max-height:none; min-height:60vh; padding:34px 24px; } .course-article { font-size:16px; line-height:1.85; } .course-section-title { font-size:19px; } }