Use floats for image background opacity

It seems percentages for opacity are still newish, and they seem to confuse
something which is clamping them to the 0 - 1 range (which makes sense for
floats, not percentages).

Anyway, for now we can get what we want here by using float values.

Fixes element-hq/element-web#17036

Author: jryans

res/themes/dark/css/_dark.scss

@@ -85,7 +85,7 @@ $dialog-close-fg-color: #9fa9ba;
 
 $dialog-background-bg-color: $header-panel-bg-color;
 $lightbox-background-bg-color: #000;
-$lightbox-background-bg-opacity: 85%;
+$lightbox-background-bg-opacity: 0.85;
 
 $settings-grey-fg-color: #a2a2a2;
 $settings-profile-placeholder-bg-color: #21262c;

res/themes/legacy-dark/css/_legacy-dark.scss

@@ -83,7 +83,7 @@ $dialog-close-fg-color: #9fa9ba;
 
 $dialog-background-bg-color: $header-panel-bg-color;
 $lightbox-background-bg-color: #000;
-$lightbox-background-bg-opacity: 85%;
+$lightbox-background-bg-opacity: 0.85;
 
 $settings-grey-fg-color: #a2a2a2;
 $settings-profile-placeholder-bg-color: #e7e7e7;

res/themes/legacy-light/css/_legacy-light.scss

@@ -127,7 +127,7 @@ $dialog-close-fg-color: #c1c1c1;
 
 $dialog-background-bg-color: #e9e9e9;
 $lightbox-background-bg-color: #000;
-$lightbox-background-bg-opacity: 95%;
+$lightbox-background-bg-opacity: 0.95;
 
 $imagebody-giflabel: rgba(0, 0, 0, 0.7);
 $imagebody-giflabel-border: rgba(0, 0, 0, 0.2);

res/themes/light/css/_light.scss

@@ -118,7 +118,7 @@ $dialog-close-fg-color: #c1c1c1;
 
 $dialog-background-bg-color: #e9e9e9;
 $lightbox-background-bg-color: #000;
-$lightbox-background-bg-opacity: 95%;
+$lightbox-background-bg-opacity: 0.95;
 
 $imagebody-giflabel: rgba(0, 0, 0, 0.7);
 $imagebody-giflabel-border: rgba(0, 0, 0, 0.2);