Set height/width to 100% in .remark-slide-content

Fixes #24

inst/resources/template.css

   background-size: {{background_size}};{{/background_size}}{{#background_position}}
   background-position: {{background_position}};{{/background_position}}{{/background_image}}
   padding: {{padding}};
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/duo-header_bg.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/duo.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/duo_accent.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/duo_accent_inverse.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/mono_accent.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/mono_accent_inverse.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/mono_dark.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/mono_light-header_bg.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/mono_light.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/solarized_dark-header_bg.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/solarized_dark.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/solarized_light.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);

tests/testthat/css/xaringan.css

   background-color: var(--background-color);
   font-size: 1rem;
   padding: 16px 64px 16px 64px;
+  width: 100%;
+  height: 100%;
 }
 .remark-slide-content h1 {
   font-size: var(--header-h1-font-size);