* {
    box-sizing:border-box;
}
<style>
html {
scroll-behavior: smooth;
}
body {
font-family: 'Helvetica Neue', Arial, sans-serif;;
}

h1, h2, h3, h4, h5, h6 {
    font-family: 'Rajdhani', sans-serif; 
}

/* ===== HEADER ===== */
header {
background-color: #6c757d;
color: white;
}

.navbar {
background-color: #6c757d;
}

.navbar-nav .nav-link {
color: white;
font-weight: 500;
transition: all 0.3s ease;
}

.navbar-nav .nav-link:hover {
text-decoration: underline;
color: #0d3791;
}

/* ===== TÍTULOS ===== */
h1.titulo {
font-size: 3rem;
color: white;
font-weight: bold;
}

h3.titulo {
font-size: 1.8rem;
color: #264994;
font-weight: bold;
margin-bottom: 1.5rem;
}

/* ===== IMAGENS ===== */
.img-iot {
  width: 300px;
  height: auto;
}

.img-termostato {
  max-width: 350px;
  height: auto;
}

/* ===== CARDS ===== */
.card {
border: 1px solid #6c757d;
box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
transition: box-shadow 0.3s ease;
}

.card:hover {
box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
}

/* ===== FOOTER ===== */
footer {
background-color: #212529;
color: white;
text-align: center;
padding: 1.5rem 0;
margin-top: 3rem;
}

/* ===== RESPONSIVIDADE ===== */
@media (max-width: 768px) {
h1.titulo {
font-size: 2rem;
}

h3.titulo {
font-size: 1.3rem;
}
}
head
    h3.titulo {
      font-size: 1.8rem; 
      color: #0957cc;
      font-weight: bold;
    }
  </style>