In the OSS Obsidian notes program: CSS snippet to make boundary between editable note area and side whitespace clearly visible by adding subtle card shadow effect and thin outline.

visible-editor-boundary.css

/* Obsidian CSS snippet for the default theme, to make the editable area clearly separated from side whitespace */

/* Adjust these if you like */
:root {
  --center-column-width: var(--file-line-width);
  /* or 800px if you want fixed */
  --center-column-padding: 24px;
}

/* Common "card" for both source and reading mode */
.markdown-source-view.mod-cm6 .cm-editor,
.markdown-reading-view .markdown-preview-section {
  position: relative;
  margin: 24px auto 32px auto;
  max-width: var(--center-column-width);
  padding: 16px var(--center-column-padding) 24px var(--center-column-padding);
  border-radius: 10px;
}

/* Light theme card look */
.theme-light .markdown-source-view.mod-cm6 .cm-editor,
.theme-light .markdown-reading-view .markdown-preview-section {
  background-color: rgba(0, 0, 0, 0.02);
  box-shadow:
    0 0 0 1px rgba(0, 0, 0, 0.12),
    0 16px 36px rgba(0, 0, 0, 0.08);
}

/* Dark theme card look */
.theme-dark .markdown-source-view.mod-cm6 .cm-editor,
.theme-dark .markdown-reading-view .markdown-preview-section {
  background-color: rgba(255, 255, 255, 0.03);
  box-shadow:
    0 0 0 1px rgba(255, 255, 255, 0.12),
    0 16px 36px rgba(0, 0, 0, 0.8);
}

/* Faint vertical rails at the exact left/right edge of the editable area */
.markdown-source-view.mod-cm6 .cm-editor::before,
.markdown-source-view.mod-cm6 .cm-editor::after,
.markdown-reading-view .markdown-preview-section::before,
.markdown-reading-view .markdown-preview-section::after {
  content: "";
  position: absolute;
  top: 8px;
  bottom: 8px;
  width: 1px;
  pointer-events: none;
  opacity: 0.4;
}

/* Left rail */
.markdown-source-view.mod-cm6 .cm-editor::before,
.markdown-reading-view .markdown-preview-section::before {
  left: 0;
  background: linear-gradient(to bottom,
      transparent,
      rgba(128, 128, 128, 0.7),
      transparent);
}

/* Right rail */
.markdown-source-view.mod-cm6 .cm-editor::after,
.markdown-reading-view .markdown-preview-section::after {
  right: 0;
  background: linear-gradient(to bottom,
      transparent,
      rgba(128, 128, 128, 0.7),
      transparent);
}