fix arrow head colors in dark mode SVGs

docs/source/finam-book/images/data-flow-adapter-dark.svg

@@ -2,7 +2,7 @@
 <svg id="svg8" width="710.47" height="133.96" version="1.1" viewBox="0 0 710.47 133.96" xmlns="http://www.w3.org/2000/svg">
   <defs id="defs39">
     <marker id="Arrow2" overflow="visible" markerHeight="5.5999999" markerWidth="7.6999998" orient="auto-start-reverse" preserveAspectRatio="xMidYMid" refX="5" viewBox="0 0 7.7 5.6">
-      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="context-stroke" fill-rule="evenodd"/>
+      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="#fff" fill-rule="evenodd"/>
     </marker>
   </defs>
   <g>

docs/source/finam-book/images/data-flow-adapter-push-dark.svg

@@ -2,7 +2,7 @@
 <svg id="svg8" width="710.47" height="133.96" version="1.1" viewBox="0 0 710.47 133.96" xmlns="http://www.w3.org/2000/svg">
   <defs id="defs39">
     <marker id="Arrow2" overflow="visible" markerHeight="5.5999999" markerWidth="7.6999998" orient="auto-start-reverse" preserveAspectRatio="xMidYMid" refX="5" viewBox="0 0 7.7 5.6">
-      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="context-stroke" fill-rule="evenodd"/>
+      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="#fff" fill-rule="evenodd"/>
     </marker>
   </defs>
   <g>

docs/source/finam-book/images/data-flow-simple-dark.svg

@@ -2,7 +2,7 @@
 <svg id="svg8" width="577.17" height="133.96" version="1.1" viewBox="0 0 577.17 133.96" xmlns="http://www.w3.org/2000/svg">
   <defs id="defs39">
     <marker id="Arrow2" overflow="visible" markerHeight="5.5999999" markerWidth="7.6999998" orient="auto-start-reverse" preserveAspectRatio="xMidYMid" refX="5" viewBox="0 0 7.7 5.6">
-      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="context-stroke" fill-rule="evenodd"/>
+      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="#fff" fill-rule="evenodd"/>
     </marker>
   </defs>
   <g>

docs/source/finam-book/images/scheduling-dark.svg

@@ -2,13 +2,13 @@
 <svg id="svg8" width="512" height="128" version="1.1" viewBox="0 0 512 128" xmlns="http://www.w3.org/2000/svg">
   <defs id="defs25">
     <marker id="marker10354" overflow="visible" markerHeight="5.5999999" markerWidth="7.6999998" orient="auto-start-reverse" preserveAspectRatio="xMidYMid" viewBox="0 0 7.7 5.6">
-      <path id="path10352" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="context-stroke" fill-rule="evenodd"/>
+      <path id="path10352" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="#fff" fill-rule="evenodd"/>
     </marker>
     <marker id="marker10007" overflow="visible" markerHeight="5.5999999" markerWidth="7.6999998" orient="auto-start-reverse" preserveAspectRatio="xMidYMid" viewBox="0 0 7.7 5.6">
-      <path id="path10005" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="context-stroke" fill-rule="evenodd"/>
+      <path id="path10005" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="#fff" fill-rule="evenodd"/>
     </marker>
     <marker id="Arrow2" overflow="visible" markerHeight="5.5999999" markerWidth="7.6999998" orient="auto-start-reverse" preserveAspectRatio="xMidYMid" viewBox="0 0 7.7 5.6">
-      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="context-stroke" fill-rule="evenodd"/>
+      <path id="arrow2L" transform="scale(.7)" d="m-2-4 11 4-11 4c2-2.33 2-5.66 0-8z" fill="#fff" fill-rule="evenodd"/>
     </marker>
   </defs>
   <g fill="#fff">