Renamed repo-img- to only- to allow setting images for only one theme (#2886)

Closes #930.

We already had classes for this kind of feature, but it was used
specifically for repository information. Just renamed the classes so it
is clear how they can be used.

To use them with markdown, just do it like this:

```markdown
![Only light](assets/img/prof_pic_color.png){: .only-light}
![Only dark](assets/img/prof_pic.jpg){: .only-dark}
```

and with html:

```html
<img
  class="only-light"
  src="assets/img/prof_pic_color.png"
>
<img
  class="only-dark"
  src="assets/img/prof_pic.jpg"
>
```

Signed-off-by: George Araújo <george.gcac@gmail.com>

_includes/repository/repo.liquid

@@ -14,12 +14,12 @@
 <div class="repo p-2 text-center">
   <a href="https://github.com/{{ include.repository }}">
     <img
-      class="repo-img-light w-100"
+      class="only-light w-100"
       alt="{{ include.repository }}"
       src="https://github-readme-stats.vercel.app/api/pin/?username={{ repo_url[0] }}&repo={{ repo_url[1] }}&theme={{ site.repo_theme_light }}&show_owner={{ show_owner }}&description_lines_count={{ max_lines }}"
     >
     <img
-      class="repo-img-dark w-100"
+      class="only-dark w-100"
       alt="{{ include.repository }}"
       src="https://github-readme-stats.vercel.app/api/pin/?username={{ repo_url[0] }}&repo={{ repo_url[1] }}&theme={{ site.repo_theme_dark }}&show_owner={{ show_owner }}&description_lines_count={{ max_lines }}"
     >

_includes/repository/repo_trophies.liquid

@@ -2,12 +2,12 @@
   <a href="https://github.com/ryo-ma/github-profile-trophy">
     <span class="d-none d-sm-none d-md-block">
       <img
-        class="repo-img-light"
+        class="only-light"
         alt="{{ include.username }}"
         src="https://github-profile-trophy.vercel.app/?username={{ include.username }}&theme={{ site.repo_trophies.theme_light }}&locale={{ site.lang }}&margin-w=15&margin-h=15&no-bg=true&rank=-C&column=6"
       >
       <img
-        class="repo-img-dark"
+        class="only-dark"
         alt="{{ include.username }}"
         src="https://github-profile-trophy.vercel.app/?username={{ include.username }}&theme={{ site.repo_trophies.theme_dark }}&locale={{ site.lang }}&margin-w=15&margin-h=15&no-bg=true&rank=-C&column=6"
       >
@@ -15,12 +15,12 @@
 
     <span class="d-none d-sm-block d-md-none">
       <img
-        class="repo-img-light"
+        class="only-light"
         alt="{{ include.username }}"
         src="https://github-profile-trophy.vercel.app/?username={{ include.username }}&theme={{ site.repo_trophies.theme_light }}&locale={{ site.lang }}&margin-w=15&margin-h=15&no-bg=true&rank=-C&column=4"
       >
       <img
-        class="repo-img-dark"
+        class="only-dark"
         alt="{{ include.username }}"
         src="https://github-profile-trophy.vercel.app/?username={{ include.username }}&theme={{ site.repo_trophies.theme_dark }}&locale={{ site.lang }}&margin-w=15&margin-h=15&no-bg=true&rank=-C&column=4"
       >
@@ -28,12 +28,12 @@
 
     <span class="d-block d-sm-none">
       <img
-        class="repo-img-light"
+        class="only-light"
         alt="{{ include.username }}"
         src="https://github-profile-trophy.vercel.app/?username={{ include.username }}&theme={{ site.repo_trophies.theme_light }}&locale={{ site.lang }}&margin-w=15&margin-h=15&no-bg=true&rank=-C&column=3"
       >
       <img
-        class="repo-img-dark"
+        class="only-dark"
         alt="{{ include.username }}"
         src="https://github-profile-trophy.vercel.app/?username={{ include.username }}&theme={{ site.repo_trophies.theme_dark }}&locale={{ site.lang }}&margin-w=15&margin-h=15&no-bg=true&rank=-C&column=3"
       >

_includes/repository/repo_user.liquid

@@ -1,12 +1,12 @@
 <div class="repo p-2 text-center">
   <a href="https://github.com/{{ include.username }}">
     <img
-      class="repo-img-light w-100"
+      class="only-light w-100"
       alt="{{ include.username }}"
       src="https://github-readme-stats.vercel.app/api/?username={{ include.username }}&theme={{ site.repo_theme_light }}&show_icons=true"
     >
     <img
-      class="repo-img-dark w-100"
+      class="only-dark w-100"
       alt="{{ include.username }}"
       src="https://github-readme-stats.vercel.app/api/?username={{ include.username }}&theme={{ site.repo_theme_dark }}&show_icons=true"
     >

_sass/_themes.scss

@@ -56,10 +56,11 @@
     display: none;
   }
 
-  .repo-img-light {
+  .only-light {
     display: block;
   }
-  .repo-img-dark {
+
+  .only-dark {
     display: none;
   }
 
@@ -111,10 +112,11 @@ html[data-theme="dark"] {
   --global-danger-block-text: #600;
   --global-danger-block-title: #c00;
 
-  .repo-img-light {
+  .only-light {
     display: none;
   }
-  .repo-img-dark {
+
+  .only-dark {
     display: block;
   }
 }