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                <div class="container">
  <h2>Lời Bác Hồ dạy các đồng chí:</h2>
    "Không có việc gì khó,
     Chỉ sợ lòng không bền,
     Đào núi và lấp biển,
     Quyết chí ắt làm nên”.


                :root {
  --blue: #1e90ff;
  --white: #ffffff; 

body {
  background-color: var(--blue);

h2 {
  border-bottom: 2px solid var(--blue);

.container {
  color: var(--blue);
  background-color: var(--white);
  padding: 15px;

button {
  --blue: #0000ff; /* biến cục bộ sẽ ghi đè toàn cục */
  background-color: var(--white);
  color: var(--blue);
  border: 1px solid var(--blue);
  padding: 5px;