Fix various JetBrains styling issues (#6153)

## Test plan

Run IntelliJ with CODY_DIR pointing to the cody repo with
`pkukielka/fix-jetbrains-styling` branch checked out.
You should notice new, improved cody panel look:

Old look:


![image](https://github.com/user-attachments/assets/ffe42c80-8bb8-4c39-b61f-17fc3093c3d2)

![image](https://github.com/user-attachments/assets/708720c1-f7f1-4dd0-af08-6202e4c94297)

![image](https://github.com/user-attachments/assets/58dd8014-c916-4735-bb3f-507f3bfbefbe)

New look:


![image](https://github.com/user-attachments/assets/39968dce-ad85-4351-a1d5-29aa79b16e12)

![image](https://github.com/user-attachments/assets/d7b26afb-50ca-48dd-b346-0c1eadbcb96e)

![image](https://github.com/user-attachments/assets/202a243c-08e2-48ae-aa24-d51445a18ca1)

vscode/webviews/themes/jetbrains.css

@@ -19,9 +19,13 @@ html[data-ide='JetBrains'] {
     --vscode-breadcrumb-foreground: var(--jetbrains-EditorPane-foreground);
     --vscode-breadcrumbPicker-background: var(--jetbrains-EditorPane-inactiveBackground);
     --vscode-button-border: var(--jetbrains-Button-startBorderColor);
+    --vscode-button-background: var(--jetbrains-Button-background);
+    --vscode-button-hoverBackground: var(--jetbrains-List-Button-hoverBackground);
     --vscode-button-foreground: var(--jetbrains-Button-foreground);
+    --vscode-button-secondaryHoverBackground: var(--jetbrains-List-Button-hoverBackground);
+    --vscode-button-secondaryBackground: var(---jetbrains-Button-background);
     --vscode-button-secondaryForeground: var(--jetbrains-Button-foreground);
-    --vscode-button-separator: var(--jetbrains-Button-shadow);
+    --vscode-button-separator: var(-jetbrains-Button-shadow);
     --vscode-charts-blue: var(--jetbrains-CheckBox-foreground);
     --vscode-charts-foreground: var(--jetbrains-ControlText);
     --vscode-charts-green: var(--jetbrains-ProgressBar-passedColor);
@@ -94,10 +98,10 @@ html[data-ide='JetBrains'] {
     --vscode-diffEditor-unchangedRegionForeground: var(--jetbrains-Table-foreground);
     --vscode-diffEditor-unchangedRegionShadow: var(--jetbrains-ScrollBar-thumbDarkShadow);
     --vscode-disabledForeground: var(--jetbrains-CheckBoxMenuItem-disabledForeground);
-    --vscode-dropdown-background: var(--jetbrains-ComboBox-background);
-    --vscode-dropdown-border: var(--jetbrains-ActionButton-hoverBorderColor);
-    --vscode-dropdown-foreground: var(--jetbrains-ComboBox-foreground);
-    --vscode-dropdown-listBackground: var(--jetbrains-ComboBox-buttonBackground);
+    --vscode-dropdown-background: var(--jetbrains-Popup-background);
+    --vscode-dropdown-border: var(--jetbrains-Popup-borderColor);
+    --vscode-dropdown-foreground: var(--jetbrains-PopupMenu-foreground);
+    --vscode-dropdown-listBackground: var(-jetbrains-Popup-background);
     --vscode-editor-background: var(--jetbrains-EditorPane-background);
     --vscode-editor-findMatchBorder: var(--jetbrains-DragAndDrop-borderColor);
     --vscode-editor-findMatchHighlightBorder: var(--jetbrains-SegmentedButton-selectedEndBorderColor);