fix(client): use sans-serif font for titles instead of monospace (#54123)

Co-authored-by: Huyen Nguyen <25715018+huyenltnguyen@users.noreply.github.com>

client/src/components/Donation/donation.css

@@ -262,8 +262,6 @@ li.disabled > a {
 .donation-modal h2 {
   font-weight: 600 !important;
   font-size: 1.2rem;
-  /* TODO: Remove this rule once https://github.com/freeCodeCamp/freeCodeCamp/issues/54047 is resolved */
-  font-family: var(--font-family-sans-serif);
 }
 
 .donation-modal b {
@@ -646,13 +644,6 @@ a.patreon-button:hover {
 }
 
 /* donation page */
-
-.donate-supporter-page-section h1,
-.donate-supporter-page-section h2,
-.donate-supporter-page-section h3 {
-  font-family: var(--font-family-sans-serif);
-}
-
 .donate-supporter-page-section h1 {
   font-size: 2.5rem;
 }

client/src/components/Footer/footer.css

@@ -45,7 +45,6 @@
 .site-footer .col-header,
 .trending-guides .col-header {
   text-align: center;
-  font-family: var(--font-family-sans-serif);
   font-weight: 700;
   font-size: 16px;
   padding: 0 15px 15px;

client/src/components/landing/landing.css

@@ -21,13 +21,7 @@
   margin-top: 0;
 }
 
-.landing-page h1,
-.landing-page h2 {
-  font-family: var(--font-family-sans-serif);
-}
-
 .landing-page .landing-page-happy {
-  font-family: var(--font-family-monospace);
   font-size: 1.1rem;
   margin-top: 0;
 }
@@ -73,7 +67,6 @@ figcaption.caption {
 .faq-question {
   font-weight: bold;
   font-size: 1.25rem;
-  font-family: var(--font-family-sans-serif);
   margin-bottom: 1.2rem;
 }
 
@@ -147,7 +140,6 @@ figcaption.caption {
 }
 
 .testimonial-meta h3 {
-  font-family: var(--font-family-sans-serif);
   font-size: 1.3rem;
   font-weight: normal;
   margin-bottom: 0;

client/src/components/layouts/global.css

@@ -75,30 +75,21 @@ h1 {
   color: var(--secondary-color);
   font-weight: 700;
   font-size: 1.5rem;
-  font-family: var(--font-family-monospace);
   margin: 0.6rem 0;
 }
 h2 {
   color: var(--secondary-color);
   font-weight: 700;
   font-size: 1.25rem;
-  font-family: var(--font-family-monospace);
   margin: 0.6rem 0;
 }
 h3 {
   color: var(--secondary-color);
   font-weight: 700;
   font-size: 1.1rem;
-  font-family: var(--font-family-monospace);
   margin: 0 0 0.6rem;
 }
 
-h4,
-h5,
-h6 {
-  font-family: var(--font-family-monospace);
-}
-
 h4,
 h5,
 h6,

client/src/templates/Introduction/intro.css

@@ -445,7 +445,6 @@ ToDo: find out why, and remove the need for it */
 .block-grid .progress-wrapper {
   width: 100%;
   text-align: left;
-  font-family: var(--font-family-sans-serif);
   display: flex;
   align-items: center;
   margin-top: 18px;