styles.css:
```css
:root {
  --primary-color: #34C759;
  --background-color: #f9f9f9;
  --text-color: #333;
  --font-family: Arial, sans-serif;
  --spacing-scale: 8px;
}

body {
  font-family: var(--font-family);
  background-color: var(--background-color);
  color: var(--text-color);
  margin: 0;
  padding: 0;
}

header {
  background-color: var(--primary-color);
  color: #fff;
  padding: var(--spacing-scale);
  text-align: center;
}

#top-bar {
  display: flex;
  justify-content: space-between;
  align-items: center;
}

#product-name {
  font-size: 24px;
  font-weight: bold;
  letter-spacing: -0.5px;
  margin-right: var(--spacing-scale);
}

#tagline {
  font-size: 14px;
  color: #fff;
}

main {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: var(--spacing-scale);
  padding: var(--spacing-scale);
}

section {
  background-color: #fff;
  border: 1px solid #ddd;
  border-radius: 10px;
  box-shadow: 0 0 10px rgba(0, 0, 0, 0.1);
  padding: var(--spacing-scale);
}

h2 {
  font-size: 18px;
  font-weight: bold;
  letter-spacing: -0.5px;
  margin-top: 0;
}

#ledger-form form {
  display: flex;
  flex-direction: column;
}

#ledger-form label {
  margin-bottom: var(--spacing-scale);
}

#ledger-form input[type="datetime-local"],
#ledger-form input[type="text"],
#ledger-form input[type="number"] {
  padding: var(--spacing-scale);
  margin-bottom: var(--spacing-scale);
  border: 1px solid #ccc;
  border-radius: 5px;
}

#submit-entry {
  background-color: var(--primary-color);
  color: #fff;
  padding: var(--spacing-scale);
  border: none;
  border-radius: 5px;
  cursor: pointer;
}

#submit-entry:hover {
  background-color: #2e9e4a;
}

#dashboard {
  display: flex;
  flex-direction: column;
}

#summary-stats {
  display: flex;
  justify-content: space-between;
  margin-bottom: var(--spacing-scale);
}

#summary-stats div {
  margin-right: var(--spacing-scale);
}

#cost-chart {
  width: 100%;
  height: 200px;
  border: 1px solid #ccc;
}

#alert-threshold form {
  display: flex;
  flex-direction: column;
}

#alert-threshold input[type="number"] {
  padding: var(--spacing-scale);
  margin-bottom: var(--spacing-scale);
  border: 1px solid #ccc;
  border-radius: 5px;
}

#set-threshold {
  background-color: var(--primary-color);
  color: #fff;
  padding: var(--spacing-scale);
  border: none;
  border-radius: 5px;
  cursor: pointer;
}

#set-threshold:hover {
  background-color: #2e9e4a;
}

#alert-banner {
  font-size: 14px;
  color: red;
}

#ledger-view table {
  border-collapse: collapse;
  width: 100%;
}

#ledger-view th,
#ledger-view td {
  border: 1px solid #ccc;
  padding: var(--spacing-scale);
  text-align: left;
}

#ledger-view th {
  background-color: #f0f0f0;
}

#ledger-table-body tr:nth-child(odd) {
  background-color: #f9f9f9;
}

#ledger-table-body tr:hover {
  background-color: #f2f2f2;
}

@media (max-width: 380px) {
  main {
    grid-template-columns: 1fr;
  }
}

@media (max-width: 1200px) {
  main {
    grid-template-columns: repeat(auto-fill, minmax(250px, 1fr));
  }
}
```