fix(NcAppContent): splitpane splitter styles with dark theme

The CSS selectors used for the splitter were no functional, so we were
using the default ones from splitpane here.

Use our variables for colors to unbreak styling in dark mode.

Signed-off-by: Jonas <jonas@freesources.org>

src/components/NcAppContent/NcAppContent.vue

@@ -418,25 +418,17 @@ export default {
  }
  }
  }
- .app-content-wrapper--vertical-split {
+
+ &.splitpanes--vertical {
  .splitpanes__splitter {
- width: 9px;
- margin-left: -5px;
- background-color: transparent;
- border-left: none;
+ background-color: var(--color-main-background);
+ border-left: 1px solid var(--color-border);
 
- &:before,
- &:after {
- display: none;
+ &::before, &::after {
+ background-color: var(--color-border);
  }
  }
  }
- .app-content-wrapper--horizontal-split {
- .splitpanes__splitter {
- height: 9px;
- margin-top: -5px;
- }
- }
 }
 
 .app-content-wrapper--show-list {